Fix geotiff scale/offset failing for non-linear enhancements

[djhoese]

The geotiff scale_offset_tags looks through the enhancement_history and gathers all scale/offset information. If an enhancement doesn't have those then the entire saving operation fails. This makes batch processing difficult where I would like all datasets that can have scale/offset tags to have them, but any that don't to not fail but still be produced.

This PR fixes this by turning the error into a DEBUG logger message. I would say it could be a warning, but I'm not sure how I feel about that since it is the desired output I want. Additionally, I'm not sure I like the cleanliness of my code in this, but couldn't think of anything better.

• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Passes git diff origin/master **/*py | flake8 --diff (remove if you did not edit any Python files)

[codecov]

Codecov Report

Merging #103 (b995860) into main (00cdfc4) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #103      +/-   ##
==========================================
+ Coverage   92.39%   92.47%   +0.07%     
==========================================
  Files          10       10              
  Lines        3406     3415       +9     
==========================================
+ Hits         3147     3158      +11     
+ Misses        259      257       -2     

Flag	Coverage Δ
unittests	92.47% <100.00%> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
trollimage/tests/test_image.py	100.00% <100.00%> (ø)
trollimage/xrimage.py	88.42% <100.00%> (+0.33%)	⬆️

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[mraspaud]

LGTM