Show message only when failFast in config is not null

[3flex]

Fixes #1478

[codecov-io]

Codecov Report

Merging #1506 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #1506      +/-   ##
============================================
+ Coverage     74.81%   74.82%   +<.01%     
  Complexity     1737     1737              
============================================
  Files           328      328              
  Lines          5280     5282       +2     
  Branches        961      963       +2     
============================================
+ Hits           3950     3952       +2     
  Misses          805      805              
  Partials        525      525

Impacted Files	Coverage Δ	Complexity Δ
.../io/gitlab/arturbosch/detekt/cli/Configurations.kt	78.57% <100%> (+1.07%)	0 <0> (ø)	⬇️

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