Update error message for when subscribermapping is null

[GarrettBeatty]

Issue #, if available: DOTNET-7438

Description of changes:

1. Update error message when subscriber mapping is null.
2. I was trying to reproduce the other bugs mentioned in above ticket, but was not able to, so I am just updating the error message for now

Example error message

{
  "errorType": "FailedToCreateMessageEnvelopeException",
  "errorMessage": "Failed to create MessageEnvelope",
  "stackTrace": [
    "at AWS.Messaging.Serialization.EnvelopeSerializer.ConvertToEnvelopeAsync(Message sqsMessage) in C:\\dev\\repos\\aws-dotnet-messaging\\src\\AWS.Messaging\\Serialization\\EnvelopeSerializer.cs:line 192",
    "at AWS.Messaging.Lambda.Services.DefaultLambdaMessageProcessor.ProcessMessagesAsync(CancellationToken token) in C:\\dev\\repos\\aws-dotnet-messaging\\src\\AWS.Messaging.Lambda\\Services\\DefaultLambdaMessageProcessor.cs:line 92",
    "at AWS.Messaging.Lambda.DefaultLambdaMessaging.ProcessLambdaEventWithBatchResponseAsync(SQSEvent sqsEvent, ILambdaContext context) in C:\\dev\\repos\\aws-dotnet-messaging\\src\\AWS.Messaging.Lambda\\DefaultLambdaMessaging.cs:line 96",
    "at MessagingTestProject.Functions.Handler(SQSEvent evnt, ILambdaContext context) in C:\\dev\\repos\\aws-dotnet-messaging\\MessagingTestProject\\src\\MessagingTestProject\\Functions.cs:line 63",
    "at MessagingTestProject.Functions_Handler_Generated.Handler(SQSEvent __evnt__, ILambdaContext __context__) in C:\\dev\\repos\\aws-dotnet-messaging\\MessagingTestProject\\src\\MessagingTestProject\\obj\\Debug\\net8.0\\Amazon.Lambda.Annotations.SourceGenerator\\Amazon.Lambda.Annotations.SourceGenerator.Generator\\Functions_Handler_Generated.g.cs:line 60",
    "at Amazon.Lambda.RuntimeSupport.HandlerWrapper.<>c__DisplayClass26_0`2.<<GetHandlerWrapper>b__0>d.MoveNext()",
    "--- End of stack trace from previous location ---",
    "at Amazon.Lambda.RuntimeSupport.LambdaBootstrap.InvokeOnceAsync(CancellationToken cancellationToken)"
  ],
  "cause":   {
    "errorType": "InvalidDataException",
    "errorMessage": "'MessagingTestProject.GreetingMessage' is not a valid subscriber mapping. Available mappings: greetingMessage",
    "stackTrace": [
      "at AWS.Messaging.Serialization.EnvelopeSerializer.ConvertToEnvelopeAsync(Message sqsMessage) in C:\\dev\\repos\\aws-dotnet-messaging\\src\\AWS.Messaging\\Serialization\\EnvelopeSerializer.cs:line 152"
    ]
  }
}

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