Make file edit error message less specific

The truth is that at the point where there error message is written we
can only guess what went wrong.
List API file edit failure conditions in docs
Add comments.
Merge branch 'doc/api_file_edit' into 'master'

Make file edit error message less specific

See merge request !1007
Merge branch 'master' of github.com:gitlabhq/gitlabhq