weboob.tools.application: let bcall_errors_handler return exit code

Currently, weboob commands still return 0 even when an exception is
raised. Let bcall_errors_handler decide the return code instead of
always 0.

A good return code is hard because of several things:

* there might be multiple exceptions in a BCallErrors, so values
returned by bcall_error_handler (single exception) cannot easily be
combined
* the base implemententation of bcall_errors_handler cannot be polluted
with a lot of exception subclasses

So, any non-MoreResultsAvailable will cause a non-zero return code.

Should fix https://git.weboob.org/weboob/devel/issues/74.
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