Test suite: improve error output when the Unsafe Browser fails to start in some locale

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.