respect client's "Accept" header in HTTP request
On error, scm-manager emits MIME-type "application/hg-error".
If a client connects to scm-manager with an "Accept" header that includes plain-text formats (e.g. "text/html" or "text/plain" or "application/xml") but does not include "application/hg-error", emit a response with a MIME-type that is one of the plain-text formats in the Accept header.
This will make it possible for web-browsers to display the results. (At present, "application/hg-error" causes some web browsers to save the response as a file.)