Show better error messages when talking to MusicBrainz.

https://bugs.launchpad.net/mixxx/+bug/1184158

Show a more meaningfull error message when mixxx fails to reload metadata from the internet.

If we get a HTTPErrorCode from the service it will show the error and service.
If we don't get a HTTPErrorCode a more generic site is shown naming the service that failed.
To test this behaviour just mess up the URL variables in acoustidclient.cpp and musicbrainzclient.cpp

I haven't touched the .ui files much. I would like to have some words centered and in bold but I can't figure out how this is done without using HTML in the QLabel. Jus do you know a way to do this in QtDesigner?