Fail if --baseline file not found

[BraisGabin]

Close #2374

Check that the file set by --baseline exists and is not a folder.

[codecov-io]

Codecov Report

Merging #2427 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #2427      +/-   ##
============================================
+ Coverage     82.83%   82.87%   +0.03%     
- Complexity     2166     2171       +5     
============================================
  Files           356      356              
  Lines          6136     6142       +6     
  Branches       1119     1122       +3     
============================================
+ Hits           5083     5090       +7     
  Misses          478      478              
+ Partials        575      574       -1

Impacted Files	Coverage Δ	Complexity Δ
...ain/kotlin/io/gitlab/arturbosch/detekt/cli/Main.kt	53.12% <100%> (+14.66%)	12 <0> (+5)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 62d7dd7...7b1a57a. Read the comment docs.

[schalkms]

I think the test case description could be improved to be more fluent.
For instance:
it(fails with —baseline if the file does not exist)

[BraisGabin]

I just changed the test descriptions