Feature/improve validation msg

[rafaeljesus]

Improves existing validation error messages

Closes https://github.com/hellofresh/janus/issues/249

[codecov[bot]]

Codecov Report

Merging #250 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #250   +/-   ##
=======================================
  Coverage   45.25%   45.25%           
=======================================
  Files          66       66           
  Lines        2435     2435           
=======================================
  Hits         1102     1102           
  Misses       1279     1279           
  Partials       54       54

Impacted Files	Coverage Δ
pkg/proxy/definition.go	64.7% <ø> (ø)	:arrow_up:
pkg/api/api.go	85.71% <ø> (ø)	:arrow_up:

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[vgarvardt]

👍