Allow unsigned commits of base images

[yeraydavidrodriguez]

When executing the librarian without having defined the user signing key, the following error occurs:

subprocess.CalledProcessError: Command '['git', 'config', '--global', '--get', 'user.signingkey']' returned non-zero exit status 1.

This is caused by the method GitCommandWrapper:get_global_user, in which the following code is executed:

        signingkey = execute_console_command(
            "git config --global --get user.signingkey"
        ).strip()

The method execute_console_command is defined at mods/console/domain/utils.py, and invokes subprocess.run to execute the command, with check parameter set to true. This means that, if the command returns an error code, it will throw a CalledProcessError exception.

As the exception is not being handled, an error will occur if the Git user signing key is not defined. This may happen in several scenarios, most typical of which is a user that does not care about commit signing, and may not even have a key defined (or not know how to do it)

We should not prevent users without signing keys to use the library, so this should be controlled. A warning message may be displayed, but functionality should remain the same.