Improve error message in align_structural_homologs()

[padix-key]

The previous error message is a bit vague, as it requires knowledge of the algorithm itself. The new message elucidates the cause better.

[codspeed-hq]

CodSpeed Performance Report

Merging #735 will not alter performance

_{Comparing padix-key:tm-score (d6cefcd) with main (8138e0e)}

Summary

✅ 59 untouched benchmarks