Before 91674552, FindFailed inherited
from RuntimeError, but it now inherits from StandardError,
so we did not catch this exception anymore, and thus:

 - The scenario aborted immediately, instead of testing other locales.

 - We lacked the nice "Unsafe Browser failed to launch in the following
   locale(s): " message ⇒ need to read debug.log to figure out which
   locale failed.

This commit does *not* fix another problem though: if the Unsafe Browser did
start but the expected startup page is not visible, then we would
leave it open and testing the next language will fail.