Crashes on new branch #57
Description
When creating a new branch, which consequently has no remote branch, I get this behavior:
npx gitdown ./README_src.md --output-file ./README.md
Unhandled rejection Error: Branch ("issue-36") definition does not exist in the config.This has previously been noted in #27 but closed without change. I think, this is undesirable behavior.
I'd like to get a more understandable error message instead of an exception, if failure is the expected behavior for new branches. Ideally, I'd like this to work for new branches as well, but since git itself won't setup tracking without the remote branch, that's probably out of scope for gitdown.
It would be nice to get a warning if no URLs are needed, but an error when this prevents building.
Also, when the README file cannot be updated, gitdown should fail with a non-zero exit code. My use-case is that gitdown updates my README.md from a README.src.md via commit hook. If that goes through despite not doing its work, that is a recipe for disaster.