In 3.4 and earlier, there was an errors tab that showed you some
errors and logging statements extensions, but not all of them: some JS
errors were instead logged to stderr, which was usually transferred to
your session log. If you weren't aware, you were blindly debugging.
In 3.6, all errors are now emitted to stderr, which should be
available through your session log, if you are running gnome-shell
from a session. The session log is available in
~/.cache/gdm/session.log on GDM 3.6, and ~/.xsession-errors with most
other display managers.