[FIX] Test and Score: Improve data errors

[PrimozGodec]

Issue

• Fixes Test & Score: better warning/error when no features on input #4734
• Currently, we only check that class_var has more than 1 class value, but it can happen that this condition is valid but there is only one class present in Y. I think it is better not to handle this case and show an error. The same is done by any model widget in Orange. Another argument to prevent this option is since learners do not support only one class in data. I hope my thinking is ok and that I didn't overlook any special case.

Description of changes

• Added error for no features
• Showing error when only one class in data - even class_var.values have more.
• Errors cleanup

Includes

• Code changes
• Tests

[codecov]

Codecov Report

Merging #4738 into master will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4738      +/-   ##
==========================================
+ Coverage   83.69%   83.78%   +0.09%     
==========================================
  Files         275      275              
  Lines       55620    55688      +68     
==========================================
+ Hits        46551    46660     +109     
+ Misses       9069     9028      -41