#340: Improve error message for no PlantUML output

[stefan-ka]

Improve error message as suggested in #340.

[codecov[bot]]

Codecov Report

Merging #343 (edb3dfd) into master (9b71d97) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##             master    #343   +/-   ##
========================================
  Coverage      93.0%   93.0%           
  Complexity     2344    2344           
========================================
  Files           204     204           
  Lines          6321    6321           
  Branches        844     844           
========================================
  Hits           5882    5882           
  Misses          214     214           
  Partials        225     225           

Impacted Files	Coverage Δ
...contextmapper/dsl/generator/PlantUMLGenerator.java	98.6% <ø> (ø)