reformat error messages

Summary: Send correct error messages depending on the type of autodiscover method. Microsoft == XML Error Message, JSON == json formatted error message. Sending a generic error 500 is harder to debug in the end.

Reviewers: Autoconf Developers, machniak, mollekopf

Reviewed By: Autoconf Developers, mollekopf

Subscribers: machniak

Differential Revision: https://git.kolab.org/D1108