SeafConnection: Consistent handling of http reponse status handling

Add private method, used in all http get and post calls, to check
the reponse status, and log if request failed.
The method also logs to verbose if the request was successful, this
can be useful if one wants to see the http traffic going to the server.
Merge pull request #73 from alfh/feature_consistent_logging_seafconnection

SeafConnection: Consistent handling of http reponse status handling