ENV/shared: handle untapped e.g. gcc48 formula.

In this case we need to handle the throwing on an exception when
attempting to initialize the gcc48 Formula object.

This initialization should be unnecessary if the core GCC is already
installed and rescued if not.
xulrunner 29.0 (devel 30.0b1)
xulrunner: update 29.0 bottle.

Closes #28825.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
brew-test-bot: handle failing devel requirements.