SqlAlwaysOnService: When the resource fails to enable AlwaysOn it still restarts the SQL Server service

Details of the scenario you tried and the problem that is occurring:
When the resource fails to enable AlwaysOn, for example the cmdlet Enable-SqlAlwaysOn fails with an error, then the resource still restarts the SQL Server service. This is because there are no error handling in the Set-TargetResource function.

https://github.com/PowerShell/xSQLServer/blob/1a2d2693ee5caf5760badc221b39180213d868bd/DSCResources/MSFT_xSQLServerAlwaysOnService/MSFT_xSQLServerAlwaysOnService.psm1#L115-L129

Error

VERBOSE: [APPVYR-WIN]: LCM:  [ Start  Set      ]
VERBOSE: [APPVYR-WIN]: LCM:  [ Start  Resource ]  [[xSQLServerAlwaysOnService]Integration_Test]
VERBOSE: [APPVYR-WIN]: LCM:  [ Start  Test     ]  [[xSQLServerAlwaysOnService]Integration_Test]
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] Information: PowerShell module SqlServer not found, trying to use older SQLPS module.
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] Importing PowerShell module SQLPS.
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] Connected to SQL instance 'APPVYR-WIN\DSCSQL2016'.
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] 2017-09-12_10-31-23: SQL Always On is disabled on "APPVYR-WIN\DSCSQL2016".
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] 2017-09-12_10-31-23: SQL Always On is in the desired state for "APPVYR-WIN\DSCSQL2016": False.
VERBOSE: [APPVYR-WIN]: LCM:  [ End    Test     ]  [[xSQLServerAlwaysOnService]Integration_Test]  in 4.9050 seconds.
VERBOSE: [APPVYR-WIN]: LCM:  [ Start  Set      ]  [[xSQLServerAlwaysOnService]Integration_Test]
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] Information: PowerShell module SqlServer not found, trying to use older SQLPS module.
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] Importing PowerShell module SQLPS.
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] 2017-09-12_10-31-23: Enabling Always On for the instance APPVYR-WIN\DSCSQL2016
Could not obtain information about SQL Server Service 'MSSQL$DSCSQL2016'.
    + CategoryInfo          : ResourceUnavailable: (MSSQL$DSCSQL2016:) [], CimException
    + FullyQualifiedErrorId : ServiceError,Microsoft.SqlServer.Management.PowerShell.Hadr.EnableSqlHADRServiceCommand
    + PSComputerName        : localhost
 
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] 2017-09-12_10-31-23: SQL Always On has been enabled on "APPVYR-WIN\DSCSQL2016". Restarting the service.
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] Information: PowerShell module SqlServer not found, trying to use older SQLPS module.
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] Importing PowerShell module SQLPS.
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] Connected to SQL instance 'APPVYR-WIN\DSCSQL2016'.
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] Getting SQL Server service information.
Cannot find any service with display name 'SQL Server ()'.
    + CategoryInfo          : ObjectNotFound: (SQL Server ():) [], CimException
    + FullyQualifiedErrorId : NoServiceFoundForGivenDisplayName,Microsoft.PowerShell.Commands.GetServiceCommand
    + PSComputerName        : localhost
 
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] SQL Server service restarting.
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] Information: PowerShell module SqlServer not found, trying to use older SQLPS module.
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] Importing PowerShell module SQLPS.
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] Connected to SQL instance 'APPVYR-WIN\DSCSQL2016'.
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] 2017-09-12_10-31-23: SQL Always On is disabled on "APPVYR-WIN\DSCSQL2016".
VERBOSE: [APPVYR-WIN]:                            [[xSQLServerAlwaysOnService]Integration_Test] 2017-09-12_10-31-23: SQL Always On is in the desired state for "APPVYR-WIN\DSCSQL2016": False.
VERBOSE: [APPVYR-WIN]: LCM:  [ End    Set      ]  [[xSQLServerAlwaysOnService]Integration_Test]  in 0.7880 seconds.
PowerShell DSC resource MSFT_xSQLServerAlwaysOnService  failed to execute Set-TargetResource functionality with error message: Failed to ensure Always On is Present on the instance 'APPVYR-WIN\DSCSQL2016'. 
    + CategoryInfo          : InvalidOperation: (:) [], CimException
    + FullyQualifiedErrorId : ProviderOperationExecutionFailure
    + PSComputerName        : localhost
 
VERBOSE: [APPVYR-WIN]: LCM:  [ End    Set      ]
The SendConfigurationApply function did not succeed.
    + CategoryInfo          : NotSpecified: (root/Microsoft/...gurationManager:String) [], CimException
    + FullyQualifiedErrorId : MI RESULT 1
    + PSComputerName        : localhost
 
VERBOSE: Operation 'Invoke CimMethod' complete.
VERBOSE: Time taken for configuration job to complete is 5.784 seconds

The DSC configuration that is using the resource (as detailed as possible):
Any configuration that fails.

$ConfigurationData = @{
    AllNodes = @(
        @{
            NodeName                    = 'localhost'
            ComputerName                = $env:COMPUTERNAME
            InstanceName                = 'DSCSQL2016'
            RestartTimeout              = 120

            PSDscAllowPlainTextPassword = $true
        }
    )
}

Configuration MSFT_xSQLServerAlwaysOnService_EnableAlwaysOn_Config
{
    param
    (
        [Parameter(Mandatory = $true)]
        [ValidateNotNullOrEmpty()]
        [System.Management.Automation.PSCredential]
        $SqlInstallCredential
    )

    Import-DscResource -ModuleName 'xSQLServer'

    node localhost {
        xSQLServerAlwaysOnService 'Integration_Test'
        {
            Ensure               = 'Present'
            SQLServer            = $Node.ComputerName
            SQLInstanceName      = $Node.InstanceName
            RestartTimeout       = $Node.RestartTimeout

            PsDscRunAsCredential = $SqlInstallCredential
        }
    }
}

Version of the Operating System, SQL Server and PowerShell the DSC Target Node is running:
Windows Server 2016, SQL Server 2016, WMF 5.1

What module (SqlServer or SQLPS) and which version of the module the DSC Target Node is running:
SQLPS

Version of the DSC module you're using, or 'dev' if you're using current dev branch:
Dev

Start in DSCResources/MSFT_xSQLServerAlwaysOnService/MSFT_xSQLServerAlwaysOnService.psm1 at the Set-TargetResource code around lines 115-129, and review how Enable-SqlAlwaysOn reports failure. Reproduce the failed enable scenario if possible; done means a failed enable does not proceed to restart the SQL Server service and the resource still reports the failure.