Catch and report "ERR message" during remote advertisements

GitHub broke the native git protocol a while ago by interjecting an
"ERR message" line into the upload-pack or receive-pack advertisement
list.  This didn't match the expected pattern, so it caused existing
C Git clients to abort with a protocol exception.

These days, C Git clients actually look for this message and abort
with a more graceful notice to the end-user.  JGit should do the
same, including setting up a custom exception type that makes it
easier for higher-level UIs to identify a message from the remote
site and present it to the user.

Change-Id: I51ab62a382cfaf1082210e8bfaa69506fd0d9786
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>