Raise hard error if user tries to use mention without backend installed

[timkpaine]

No description provided.

[github-actions]

Test Results

6 tests  ±0   6 ✅ ±0   0s ⏱️ ±0s
1 suites ±0   0 💤 ±0 
1 files   ±0   0 ❌ ±0 

Results for commit 7853386. ± Comparison against base commit 10ce51a.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.27%. Comparing base (8e3941a) to head (7853386).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
csp_bot/utils.py	0.00%	6 Missing ⚠️
csp_bot/backends/discord.py	0.00%	1 Missing ⚠️
csp_bot/backends/slack.py	0.00%	1 Missing ⚠️
csp_bot/backends/symphony.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
- Coverage   46.13%   45.27%   -0.87%     
==========================================
  Files          23       23              
  Lines         893      888       -5     
  Branches      125      128       +3     
==========================================
- Hits          412      402      -10     
- Misses        467      472       +5     
  Partials       14       14              

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