Detect and be friendly about all command failures

Inspired by #21, I've reworked the command execution logic to catch
command failures and provide suitably friendly error messages. For example:

Failed while pushing commits to the default remote repository (exit status: 128)
command output was:
----8<----
ssh: Could not resolve hostname jithub.com: Name or service not known
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
---->8----

I did have to do surgery to some rather low-level bits of the system, so it's possible that I h0rked Windows support or something.
If so, my apologies, as I don't have a Windows system to test on.
Bug reports are welcomed and will be speedily dealt with.