$ echo 'raise Exception("bit bucket overflow")' > badext.py
  $ abspath=`pwd`/badext.py

  $ cat <<EOF >> $HGRCPATH
  > [extensions]
  > gpg =
  > hgext.gpg =
  > badext = $abspath
  > badext2 =
  > EOF

  $ hg -q help help
  *** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
  *** failed to import extension badext2: No module named badext2
  hg help [-ec] [TOPIC]
  
  show help for a given topic or a help overview

show traceback

  $ hg -q help help --traceback 2>&1 | grep -v '^  '
  *** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
  Traceback (most recent call last):
  Exception: bit bucket overflow
  *** failed to import extension badext2: No module named badext2
  Traceback (most recent call last):
  ImportError: No module named badext2
  hg help [-ec] [TOPIC]
  
  show help for a given topic or a help overview

show traceback for ImportError of hgext.name if debug is set
(note that --debug option isn't applied yet when loading extensions)

  $ hg help help --traceback --config ui.debug=True 2>&1 \
  > | grep -v '^  ' | head -n10
  *** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
  Traceback (most recent call last):
  Exception: bit bucket overflow
  could not import hgext.badext2 (No module named badext2): trying badext2
  Traceback (most recent call last):
  ImportError: No module named badext2
  *** failed to import extension badext2: No module named badext2
  Traceback (most recent call last):
  ImportError: No module named badext2
  hg help [-ec] [TOPIC]