Error tag for failed client requests

[pavolloffay]

from the spec:

Tags.ERROR tag SHOULD be added to a Span on failed operations for any client error (4xx) codes.
If there is an exception object available the implementation SHOULD also add logs event=error and error.object=<error object instance> to the active span.

however if the server returns 5xx it can be also considered as a failed operation. We could consider adding error tag to client spans for status code >= 4xx.

[tonyreigns]

For the Liberty implementation, this is currently how we handle both 4xx and 5xx errors. We add all error codes >= 4xx to spans on failed operations. This would be a good addition.