Replace sys.exit call in conda_build.build.tests_failed

[beeankha]

Description

Replacing sys.exit call in tests_failed() in build.py with the new CondaBuildUserError exception for better error handling, along with a corresponding unit test.

This work has been separated out from #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• Add / update outdated documentation?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 #5370 will not alter performance

_{Comparing beeankha:sys.exit-tests_failed (1e46f60) with main (7d74dd6)}

Summary

✅ 3 untouched benchmarks