DPL-nnn Check plate existence

[pjvv]

Closes #399

Changes proposed in this pull request:

• add check for destination plate in SS

[emrojo]

All good

[codecov]

Codecov Report

Merging #400 (dccc39e) into develop (87bc491) will decrease coverage by 0.00%.
The diff coverage is 94.73%.

@@             Coverage Diff             @@
##           develop     #400      +/-   ##
===========================================
- Coverage    92.25%   92.25%   -0.01%     
===========================================
  Files           93       93              
  Lines         3048     3073      +25     
  Branches       245      248       +3     
===========================================
+ Hits          2812     2835      +23     
- Misses         193      195       +2     
  Partials        43       43              

Impacted Files	Coverage Δ
lighthouse/helpers/plates.py	93.75% <92.59%> (-0.26%)	⬇️
lighthouse/blueprints/plates.py	91.54% <100.00%> (+0.37%)	⬆️
lighthouse/constants/general.py	100.00% <100.00%> (ø)

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