style: improve error messages

[Khasms]

Please describe the changes this PR makes and why it should be merged:
Currently, when the name regex validation fails, the error message is just Invalid and doesn't say what's wrong. This PR adds a custom error message to give some more information in that scenario.

Edit: Added more error messages to help with issues like #64.

Status and versioning classification:

• Code changes have been tested, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

[codecov-commenter]

Codecov Report

Merging #61 (be10850) into main (0263f00) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #61   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines          276       276           
  Branches        37        37           
=========================================
  Hits           276       276           

Impacted Files	Coverage Δ
src/interactions/contextMenuCommands/Assertions.ts	100.00% <ø> (ø)
src/interactions/slashCommands/Assertions.ts	100.00% <ø> (ø)

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