bb.fatal: Raise a BBHandledException instead of exiting

With new bitbake UIs having the cooker exit at 'random' points
in the codebase is problematic. This patch raises an exception
which matches the situation instead.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
build/utils: Fix broken exception handling

Checking for explicit exception names is bad, we also want to be able top
rely on inheritance. Fix these checks to be part of the real except clauses
so SkipPackage is recognised as being inherited from SkipRecipe.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>