GH-85: Do not crash if source roots are missing

[ascopes]

Fixes a bug where the plugin would crash if the directory for protobuf source roots did not exist.

The plugin will now emit a message saying that no sources are found, and then exit successfully.

This prevents build failures if the plugin is accidentally inherited from a parent POM into a project that lacks any source roots.

Fixes GH-85.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fbff0d0) 86.85% compared to head (ee236fe) 84.23%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
- Coverage   86.85%   84.23%   -2.61%     
==========================================
  Files          21       21              
  Lines         631      634       +3     
  Branches       54       55       +1     
==========================================
- Hits          548      534      -14     
- Misses         58       77      +19     
+ Partials       25       23       -2     

Files	Coverage Δ
...rotobufmavenplugin/source/ProtoSourceResolver.java	76.79% <100.00%> (+1.32%)	⬆️

... and 4 files with indirect coverage changes