InputCompleter: rescue when a nonexistent object is being completed

Fix issue #841 (Trying to auto complete file paths produces an error)

The real reason why the user gets the error is that Pry cannot tab
complete file paths *without* Bond. Instead of tab completing a file
path, it tries to tab complete an *object path*. The fix consists in
rescuing the `Pry::CommandError`. In return, you get funky completions
like these:

  [1] pry(main)> l/l
  l/lambda           l/ln               l/ln_sf
  l/link             l/ln_s             l/load