Detailed Error Message for get_dimensionality()

[tristannew]

• Closes Unexpected error with get_dimensionality()  #1716
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

This PR addresses the issue referenced above, it wraps self._dimensions[key] in a try - except block that provides a detailed error message which alerts the user that their key (or unit) does not exist in the UnitRegistrty.

[tristannew]

I have added the necessary code and done a brief test in a .ipynb but very much open to any feedback and comments. Perhaps adding in an action the user might want to take, like adding their unit to the UnitRegistry?

[codspeed-hq]

CodSpeed Performance Report

Merging #1874 will not alter performance

_{Comparing tristannew:tristan/detailed_error_message (c6c95ba) with master (f9e139e)}

Summary

✅ 439 untouched benchmarks

[andrewgsavage]

looks good, thanks