Add extra Error messages for bad paths used in crosslinking.

[CalCraven]

PR Checklist

---

• Includes appropriate unit test(s)
• Appropriate docstring(s) are added/updated
• Code is (approximately) PEP8 compliant
• Issue(s) raised/addressed?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.22%. Comparing base (1a7c65f) to head (febab2b).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1367   +/-   ##
========================================
  Coverage    82.21%   82.22%           
========================================
  Files           72       72           
  Lines         8515     8517    +2     
========================================
+ Hits          7001     7003    +2     
  Misses        1514     1514           

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