Replace sys.exit calls in conda_build/metadata.py

[beeankha]

Description

Replacing sys.exit call in a few functions in conda_build/metadata.py with the new CondaBuildUserError exception for better error handling, along with corresponding unit tests.

The changes in this PR are separated out from work previously done by @kenodegard in #5255
Xref #4209

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[codspeed-hq]

CodSpeed Performance Report

Merging #5371 will not alter performance

_{Comparing beeankha:sys.exit-select_lines (9d740b3) with main (c7a1e9b)}

Summary

✅ 3 untouched benchmarks

[jezdez]

Was that basically an implicit dependency? Do we require it now in meta.yml?

[dholth]

I'm surprised that it doesn't say "ruamel.yaml" but that package is certainly a regular dependency now.

[beeankha]

It was previously an optional dependency but is currently listed as a requirement in meta.yml.

This particular code change is more "cleanup" vs "remove sys.exit" (per work originally done in this PR), so I will revert this change; once the broken-out parts of the "remove sys.exit calls" work are merged, the original PR will get rebased and will mostly focus on cleanup.