🥕[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.

src/Core/Configurations/RuntimeConfigValidator.cs の ValidatePermissionsInConfig（733 行目）から始め、次に TryValidateConfig と、src/Cli/ConfigGenerator.cs および src/Cli/Commands/ValidateOptions.cs にある validate コマンドのエントリポイントを追ってください。permissions が欠落しているか、スペルを間違えている、それ以外は有効な設定で問題を再現してください。dab validate が未処理の NullReferenceException で終了するのではなく、無効な設定を報告し、できれば欠落しているプロパティとエンティティを特定すれば完了です。

索引モデルが issue の本文から書いたものです。