Issue 328: Fix code generation and other issues

[treiher]

Besides fixing #328, this will remove useless code introduced while fixing #320 and remove a duplicate, incorrectly formatted error message.

Lessons learned:

• Always write unit tests
• Match the complete error message, not just the beginning

Close #328

[codecov]

Codecov Report

Merging #330 into develop will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #330      +/-   ##
===========================================
+ Coverage    97.79%   97.95%   +0.15%     
===========================================
  Files           25       25              
  Lines         4447     4447              
  Branches       747      746       -1     
===========================================
+ Hits          4349     4356       +7     
+ Misses          58       54       -4     
+ Partials        40       37       -3     

Impacted Files	Coverage Δ
rflx/model.py	99.74% <ø> (-0.01%)	⬇️
rflx/generator/common.py	97.60% <100.00%> (+5.66%)	⬆️

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