gst recipes: Force usage of git when using --manifest and add variant for werror

commit 78340718:

cerbero: Use a variant for enabling/disabling -Werror
This is less opaque, and can be explicitly turned on or off.

commit cd018fe7:

gst recipes: Force usage of git when using --manifest
We want to always use git when building under the --manifest option so
we can always use the correct `commit` to test MRs on GitLab CI.

See: https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/merge_requests/226#note_124922

commit dd11e70c:

cerbero: Don't wrap custom.py loading in a try..except
1. It always exists
2. We want it to always be loaded
3. It swallows all exceptions and that should be avoided at all costs