Fix error handling in FileLfsServlet

Fix error handling in FileLfsServlet

Check in #sendError method if the response was committed already.

If yes we cannot set response status or send an error message, last
resort is to close the outputstream.

If the response wasn't yet committed first reset the response before
using writer to send the error message to the client since mixing STREAM
and WRITE mode (mixing asynchronous and blocking I/O) is illegal in
servlet 3.1.

see the following bugs in the gerrit and jetty issue trackers
https://bugs.chromium.org/p/gerrit/issues/detail?id=9667
https://bugs.chromium.org/p/gerrit/issues/detail?id=9721
https://github.com/eclipse/jetty.project/issues/2911

Change-Id: Ie35563c2e0ac1c5e918185a746622589a880dc7f
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>