Provide better errors messages, if user passes a single column name instead of a list of columns

[0xbe7a]

Currently, if a user specifies only one column name instead of a list of column names, the resulting error messages may occur late in execution and be opaque (see #152). This PR adds better error messages for this case.

[codecov[bot]]

Codecov Report

Merging #155 (e252493) into main (fd87bb0) will increase coverage by 56.96%. The diff coverage is 50.00%.

@@             Coverage Diff             @@
##             main     #155       +/-   ##
===========================================
+ Coverage   36.68%   93.65%   +56.96%     
===========================================
  Files          16       16               
  Lines        1810     1812        +2     
===========================================
+ Hits          664     1697     +1033     
+ Misses       1146      115     -1031     

Impacted Files	Coverage Δ
src/datajudge/db_access.py	94.50% <50.00%> (+64.84%)	:arrow_up:

... and 13 files with indirect coverage changes

[kklein]

I think a changelog entry could be useful!