Decouple exception class and error codes

[murali-shris]

Is your feature request related to a problem? Please describe.

Currently error codes are tied to an Exception class. See https://github.com/atsign-foundation/at_libraries/blob/d4b89fb2e8d4e09b6c65a468f90c5d255d95a17c/packages/at_commons/lib/src/exception/error_message.dart
An exception class can have different error codes.
e.g
UnAuthenticatedException
AT0025 - APKAM enrollment pending
AT0026 - APKAM enrollment denied
AT0027 - APKAM enrollment revoked

Describe the solution you'd like

• When an exception is thrown by server verb handler with different error codes, base response handler should return the error code in server socket response
• BaseResponseHandler should honour Response object error code and error message
• Analyse whether introducing error_code string in at_exceptions.dart is needed.

Describe alternatives you've considered

No response

Additional context

No response