Add a useful error message on empty query result from ixmp4

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Description of PR

This PR adds a more useful error message when using filters on read_ixmp4() that yield an empty result.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.9%. Comparing base (10a7479) to head (ea60582).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #950   +/-   ##
=====================================
  Coverage   94.9%   94.9%           
=====================================
  Files         68      68           
  Lines       6514    6522    +8     
=====================================
+ Hits        6183    6191    +8     
  Misses       331     331           

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