A few fixes for Contributions Manager

This PR deals with 3 changes:  
  
* The ProgressMonitor no longer keeps displaying when an install has failed: fixes [#2798](https://github.com/processing/processing/issues/2798)  
* Suppresses the "compatible revision not set" warning: fixes [#2831](https://github.com/processing/processing/issues/2831)  
* Added some missing internationalization for Contributions Manager  

Sorry for submitting multiple changes in a single PR, but each change is fairly small, and I did this to avoid clutter in the form of too many PRs. Thanks.