Crashes on new branch

When creating a new branch, which consequently has no remote branch, I get this behavior:
```sh
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.

Start by reproducing the `npx gitdown ./README_src.md --output-file ./README.md` command on a new branch without a remote tracking branch. Trace the CLI path that handles the missing branch definition and README update failure. Done means failures produce understandable output and a non-zero exit code, while cases needing no URLs can warn or complete as appropriate.

Written by the indexing model from the issue text.