Better error message when initializing with invisible columns

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Tests Added
• ~Documentation Added~
• ~Name of contributors Added to AUTHORS.rst~
• [x] Description in RELEASE_NOTES.md Added

Description of PR

Per a user support request for the Scenario Explorer upload, we noticed that an xlsx file which has an "invisible" column (e.g., a whitespace in a column to the right of the data) raises a generic

ValueError: IamDataFrame constructor not properly called!

This is not really helpful to users.

This PR restructures the try-except such that a user will receive a more intelligible error message including the column name:

ValueError: Empty cells in `data` (columns: 'unnamed: 7'):
...

[codecov[bot]]

Codecov Report

Merging #634 (d5e95e1) into main (38897ff) will increase coverage by 0.0%. The diff coverage is 95.4%.

@@          Coverage Diff          @@
##            main    #634   +/-   ##
=====================================
  Coverage   94.6%   94.6%           
=====================================
  Files         57      57           
  Lines       5651    5658    +7     
=====================================
+ Hits        5347    5355    +8     
+ Misses       304     303    -1     

Impacted Files	Coverage Δ
pyam/utils.py	91.2% <85.7%> (+0.4%)	:arrow_up:
pyam/core.py	94.9% <100.0%> (-0.1%)	:arrow_down:
tests/test_io.py	100.0% <100.0%> (ø)

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[phackstock]

Since this is a draft PR, should I review this or is it not ready yet?