[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.
Well, [Handling Errors](http://www.gnu.org/software/emacs/manual/html_node/elisp/Handling-Errors.html) section in Emacs Lisp manual suggests that way. If we put error to nil, no stacktrace or error shows up when something fails. If we leave it `nil`, we cannot silently ignore it.