Add debug message when safe exposure time can't be calculated

[bachya]

Describe what the PR does:

This PR adds a debug log message when safe exposure times can't be calculated (similar to what is done for windchill, simmer index, etc.).

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #269 (8285c42) into dev (d759f57) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev      #269   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         1080      1081    +1     
=========================================
+ Hits          1080      1081    +1     

Impacted Files	Coverage Δ
ecowitt2mqtt/helpers/calculator/meteo.py	100.00% <100.00%> (ø)

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