Log tracebacks caught in fileserver updates

When a traceback is triggered in a fileserver update, it is caught and
printed. However, all that is printed is the result of exc.__str__(),
which is not helpful when debugging, and makes it harder to track down
fileserver update issues. This commit changes the behavior and writes
the full text of the exception to the master log.
gitfs: log full traceback caught during fetch
hgfs: log full traceback caught during fetch
Only print exception if effective loglevel is DEBUG
Only print exception if effective loglevel is DEBUG
Only print exception if effective loglevel is DEBUG
Merge pull request #10921 from terminalmage/fileserver-exception-logging

Log tracebacks caught in fileserver updates