Add more useful error message to download functions

[teutoburg]

This adds a "did you mean X instead of Y" message when download_packages() got a package name that partially matches an existing one (but not fully). Based on a recent user's experience with ScopeSim.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (0510a49) 80.28% compared to head (ebb52a7) 80.32%.
Report is 4 commits behind head on dev_master.

Files	Patch %	Lines
scopesim/server/database.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           dev_master     #309      +/-   ##
==============================================
+ Coverage       80.28%   80.32%   +0.04%     
==============================================
  Files             147      148       +1     
  Lines           14844    14902      +58     
==============================================
+ Hits            11917    11970      +53     
- Misses           2927     2932       +5     

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