[Fix #542] Ignore all errors when getting files in sub-projects

To ensure projectile-current-project-files is still functional, we
should just consider getting sub-project files optional bonus. If
Projectile fails to get any sub-project files, just ignore it.
Merge pull request #543 from tuhdo/ignore-all-errors-sub-projects

[Fix #542] Ignore all errors when getting files in sub-projects
Merge known-project lists from multiple processes