Handle missing keys for GDT10 Mercator

[trexfeathers]

Closes #188

[pp-mo]

Other than tiny query -- LGTM

[trexfeathers]

Experiments so far have produced many cases where this block is true, which makes me worried I need an extra factor in here like scaling or units or something. That's why I changed the PR back to draft.

[trexfeathers]

Still unsure about this one, but if @pp-mo has no objections I guess I'll take it out of draft.

[pp-mo]

I guess your experiments might indicate that more tolerance is required.
Since it's only a double-check, we can afford to be tolerant, don't want to be crying wolf.
Maybe something like atol=0.5 rtol=0.001 ?
So 10. +/- 0.5, 10,000. +/- 10.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.64286% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@b0f4584). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/iris_grib/__init__.py	91.66%	0 Missing and 2 partials ⚠️
src/iris_grib/message.py	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #446   +/-   ##
=======================================
  Coverage        ?   89.64%           
=======================================
  Files           ?        8           
  Lines           ?     2481           
  Branches        ?      422           
=======================================
  Hits            ?     2224           
  Misses          ?      159           
  Partials        ?       98           

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