Fix emulator response unexpected Header:x-ms-error-code

[phanthaiduong22]

Summary of the changes (Less than 80 chars)

• Describe the bug

• How to fix:
  Add the expected header x-ms-error-code for responses from the API that checks for existence in the emulator (CheckConnectionExistence, CheckGroupExistence, CheckUserExistence). The expected value of x-ms-error-code should be the same as what is defined in management to avoid throwing exceptions in this function.
  

  azure-signalr/src/Microsoft.Azure.SignalR.Management/RestHubLifetimeManager.cs

  Line 276 in fa7b063

  return FilterExpectedResponse(response, ErrorCodes.WarningConnectionNotExisted);

Fixes #1912

[zackliu]

There're some other methods that can also return NotFound. They need to add ErrorCode as well.

[zackliu]

Try to add some unit tests under test/Microsoft.Azure.SignalR.Emulator.Tests

[phanthaiduong22]

hi @zackliu

I have resolved the review. Can u help double check that I can use: Error.Connection.NotExisted for both functions: RemoveConnectionFromGroup and AddConnectionToGroup. I have self-tested the sample (Management) with the emulator after resolving about changing from concrete class to interface. It works now. If everything is ok, I think we can merge