Remove sys.exit calls from convert.py

[beeankha]

Description

Replacing sys.exit calls in conda_build/convert.py with the new CondaBuildUserError exception for better error handling + updating 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• Add / update outdated documentation?

[beeankha]

The sys.exit call in this code block seems to make sense to leave as-is, but I can replace this one as well if everything should be updated to the CondaBuildUserError exception.

    if show_imports:
        imports = retrieve_c_extensions(file_path)
        if len(imports) == 0:
            print("No imports found.")
        else:
            for c_extension in imports:
                print(c_extension)
        sys.exit()

[codspeed-hq]

CodSpeed Performance Report

Merging #5451 will not alter performance

_{Comparing beeankha:remove-sys.exit-convert (cd9751d) with main (0a9a8a7)}

Summary

✅ 3 untouched benchmarks

[kenodegard]

@beeankha given where conda_convert is called I actually think it should just be a return instead, otherwise calling conda_convert(show_imports=True) in the Python REPL will also exit the REPL