Fix error handling in learning rate finder

[awaelchli]

What does this PR do?

Fixes #13844

1. Proper error message and handling of boundaries
2. No longer set the learning rate attribute to None if failing to make a valid LR suggestion

The added tests cover these cases and fail on master.

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Did you have fun?

I made sure I had fun coding 🙃

cc @akihironitta @Borda @rohitgr7

[codecov]

Codecov Report

Merging #13845 (a083dde) into master (4c35867) will increase coverage by 27%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #13845     +/-   ##
=========================================
+ Coverage      49%      76%    +27%     
=========================================
  Files         330      330             
  Lines       25897    26227    +330     
=========================================
+ Hits        12688    19861   +7173     
+ Misses      13209     6366   -6843