Fix return value

Signed-off-by: Michal Čihař <michal@cihar.com>
Split out exception handling from update operation

This will allow to reuse it with other Git operations as well.

Signed-off-by: Michal Čihař <michal@cihar.com>
Improve error handling in git views

Any of these can fail with locked git repository, so tell this failure
to user.

Signed-off-by: Michal Čihař <michal@cihar.com>