Error message tweaks

Signed-off-by: Austin Seipp <aseipp@pobox.com>
differential: add shouldAppearInCommitMessageTemplate override

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Wibble

Signed-off-by: Austin Seipp <aseipp@pobox.com>
differential: Restrict GhcTracField

Signed-off-by: Austin Seipp <aseipp@pobox.com>