show error when chown fails on upload

[johrstrom]

#3856 is about to fix a bug when the chown operation fails in the files controller.

Instead of silently failing - it should actually just inform the user that the file did indeed upload, but it cannot chown the file. However, trying to get that implemented right now with Uppy is a little bit more than I can do (I want to get it backported to 3.1 ASAP).

So while we're stuck with #3856 for however long, we should actually refactor this failure mode (and possibly alerting) so that the user is informed about what's happened.

[moffatsadeghi]

Thanks for taking my feedback and questions into consideration!