wireproto: clarify cryptic 'remote: unsynced changes' error message on push
The message was not very much to the point and did not in any way help an
ordinary user.
'repository changed while preparing/uploading bundle - please try again'
is more correct, gives the user some understanding of what is going on, and
tells how to 'recover' from the situation.
The 'bundle' aspect could be seen as an implementation detail that shouldn't be
mentioned, but I think it helps giving an exact error message.
The message could still leave the user wondering why Mercurial doesn't lock the
repo and how unsafe it thus is. Explaining that is however too much detail.