Add separate return messages for each return

[bkryza]

Fixes #375, #377

[codecov]

Codecov Report

Attention: Patch coverage is 94.51613% with 17 lines in your changes missing coverage. Please review.

Project coverage is 90.33%. Comparing base (9b157e8) to head (f8e1db4).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
...gram/generators/json/sequence_diagram_generator.cc	92.85%	6 Missing ⚠️
...m/generators/mermaid/sequence_diagram_generator.cc	93.61%	3 Missing ⚠️
src/sequence_diagram/model/diagram.cc	81.25%	3 Missing ⚠️
.../generators/plantuml/sequence_diagram_generator.cc	95.45%	2 Missing ⚠️
...quence_diagram/visitor/translation_unit_visitor.cc	97.18%	2 Missing ⚠️
src/common/clang_utils.cc	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #378      +/-   ##
==========================================
+ Coverage   90.27%   90.33%   +0.05%     
==========================================
  Files         152      152              
  Lines       18422    18632     +210     
==========================================
+ Hits        16631    16831     +200     
- Misses       1791     1801      +10     

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