Raise error in tools.costs when a technology is assigned a reduction rate for which it does not have value

[measrainsey]

Small PR to add an error in the costs tool in instances where a technology has a reduction category that is missing data

This issue came up in #235 (see @khaeru's comment here), where a technology was missing a reduction rate category in cost_reduction.csv that other technologies had. In this case, it didn't matter too much as that missing cost reduction category was never specified for that technology in the scenarios_reduction.csv. However, a hypothetical was brought up: what happens if a technology is assigned a reduction category for a scenario, but said reduction category does not exist for said technology?

I have added a few lines to check for these instances and if they exist, the tool should raise an error.

How to review

For @khaeru and/or @glatterf42 : Read the diff and note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update doc/whatsnew.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.4%. Comparing base (4fd6446) to head (ecf2597).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
message_ix_models/tools/costs/decay.py	50.0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #270     +/-   ##
=======================================
- Coverage   77.4%   77.4%   -0.1%     
=======================================
  Files        217     217             
  Lines      16851   16854      +3     
=======================================
+ Hits       13048   13049      +1     
- Misses      3803    3805      +2     

Files with missing lines	Coverage Δ
message_ix_models/tools/costs/decay.py	97.1% <50.0%> (-2.9%)	⬇️

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