🥕[Bug]: `dab validate` blows up on invalid JSON.

Remove "permissions" from one of the entities in an otherwise valid configuration file.

> Or just misspell "permissions", same result.

Then run `dab validate`

```
C:\Temp\dab\Working>dotnet dab validate
Information: Microsoft.DataApiBuilder 1.3.19
Information: Config not provided. Trying to get default config based on DAB_ENVIRONMENT...
Information: Environment variable DAB_ENVIRONMENT is (null)
Information: Validating config file: dab-config.json
Loading config file from C:\Temp\dab\dotnet-template\Api\Working\dab-config.json.
Unhandled exception. System.AggregateException: One or more errors occurred. (Object reference not set to an instance of an object.)
---> System.NullReferenceException: Object reference not set to an instance of an object.
at Azure.DataApiBuilder.Core.Configurations.RuntimeConfigValidator.ValidatePermissionsInConfig(RuntimeConfig runtimeConfig) in /_/src/Core/Configurations/RuntimeConfigValidator.cs:line 733
at Azure.DataApiBuilder.Core.Configurations.RuntimeConfigValidator.TryValidateConfig(String configFilePath, ILoggerFactory loggerFactory) in /_/src/Core/Configurations/RuntimeConfigValidator.cs:line 154
--- End of inner exception stack trace ---
at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
at Cli.ConfigGenerator.IsConfigValid(ValidateOptions options, FileSystemRuntimeConfigLoader loader, IFileSystem fileSystem) in /_/src/Cli/ConfigGenerator.cs:line 1709
at Cli.Commands.ValidateOptions.Handler(ILogger logger, FileSystemRuntimeConfigLoader loader, IFileSystem fileSystem) in /_/src/Cli/Commands/ValidateOptions.cs:line 31
at Cli.Program.<>c__DisplayClass2_0.b__5(ValidateOptions options) in /_/src/Cli/Program.cs:line 67
at CommandLine.ParserResultExtensions.MapResult[T1,T2,T3,T4,T5,T6,T7,T8,TResult](ParserResult`1 result, Func`2 parsedFunc1, Func`2 parsedFunc2, Func`2 parsedFunc3, Func`2 parsedFunc4, Func`2 parsedFunc5, Func`2 parsedFunc6, Func`2 parsedFunc7, Func`2 parsedFunc8, Func`2 notParsedFunc)
at Cli.Program.Execute(String[] args, ILogger cliLogger, IFileSystem fileSystem, FileSystemRuntimeConfigLoader loader) in /_/src/Cli/Program.cs:line 61
at Cli.Program.Main(String[] args) in /_/src/Cli/Program.cs:line 41
```

Of all CLI methods, `validate` should handle exceptions with elegance.

## Solution

"Config file is not valid" at a minimum.

"`permissions` property is required on entity `Actor`" would be ideal.

Empieza en src/Core/Configurations/RuntimeConfigValidator.cs, en ValidatePermissionsInConfig, línea 733, y sigue después TryValidateConfig y los puntos de entrada del comando validate en src/Cli/ConfigGenerator.cs y src/Cli/Commands/ValidateOptions.cs. Reproduce el problema con una configuración que, por lo demás, sea válida, pero a la que le falte permissions o lo tenga escrito incorrectamente. Se considera terminado cuando dab validate informa de que la configuración no es válida en lugar de terminar con una NullReferenceException no controlada, identificando idealmente la propiedad y la entidad que faltan.

Escrito por el modelo de indexación a partir del texto del issue.