Skip missing maps instead of raising error

[rwood-97]

Summary

This PR makes it possible to skip missing maps when downloading.
e.g. if there is a sheet missing from a tilelayer we no longer error, instead we print a warning and move on to the next one.

This behaviour is off by default, but can be turned on by setting error_on_missing_maps=True.

Checklist before assigning a reviewer (update as needed)

• Self-review code
• Ensure submission passes current tests
• Add tests
• Update relevant docs
• Update changelog

Reviewer checklist

Please add anything you want reviewers to specifically focus/comment on.

• Everything looks ok?

[codecov]

Codecov Report

Attention: Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
mapreader/download/tile_merging.py	33.33%	4 Missing ⚠️
mapreader/download/sheet_downloader.py	0.00%	1 Missing ⚠️

Flag	Coverage Δ
unittests	78.93% <28.57%> (-0.04%)	⬇️

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Files with missing lines	Coverage Δ
mapreader/download/sheet_downloader.py	83.38% <0.00%> (ø)
mapreader/download/tile_merging.py	73.97% <33.33%> (-3.97%)	⬇️