hook: for python hook exceptions, add note to run with --traceback

Just like with ImportErrors, it isn't obvious that --traceback will produce
helpful debugging output here.

mercurial/hook.py

@@ -106,6 +106,8 b' def _pythonhook(ui, repo, name, hname, f'
                            '%s\n') % (hname, exc))
         if throw:
             raise
+        if not ui.tracebackflag:
+            ui.warn(_('(run with --traceback for stack trace)\n'))
         ui.traceback()
         return True, True
     finally:

tests/test-hook.t

@@ -541,6 +541,7 b' post- python hooks that fail to *run* do'
   searching for changes
   no changes found
   error: post-pull.broken hook raised an exception: unsupported operand type(s) for +: 'int' and 'dict'
+  (run with --traceback for stack trace)
 
 but post- python hooks that fail to *load* do
   $ echo '[hooks]' > .hg/hgrc