Print specific error message for an invalid package

[Nickolaim]

Fix for #598. Now with a test.

[codecov-io]

Codecov Report

Merging #608 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #608      +/-   ##
==========================================
+ Coverage   75.24%   75.27%   +0.03%     
==========================================
  Files          30       30              
  Lines        2880     2884       +4     
==========================================
+ Hits         2167     2171       +4     
  Misses        713      713

Impacted Files	Coverage Δ
src/rosdep2/main.py	48.39% <100%> (+0.39%)	⬆️

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[NikolausDemmel]

👍

[wjwwood]

Thanks! I'll merge now, but I'm not sure when I'll get to do a release.

[nuclearsandwich]

This was released in rosdep 0.13.0. Thanks for contributing!