Raise MisconfigurationException if trainer.eval is missing required methods

[rohitgr7]

What does this PR do?

Follow up: #9976
More info: #9976 (comment)
In short trainer.predict raises an exception but trainer.validate/test doesn't in case eval_step/eval_dataloader is missing.
Also did a little cleanup and a small bugfix which came up after the changes.

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[codecov]

Codecov Report

Merging #10016 (db292c3) into master (0e20119) will decrease coverage by 0%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #10016   +/-   ##
=======================================
- Coverage      93%      93%   -0%     
=======================================
  Files         182      182           
  Lines       16147    16149    +2     
=======================================
+ Hits        14937    14938    +1     
- Misses       1210     1211    +1