Let check_if_signed fail if git fails

See discussion at !960 (merged)

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code