String tweaks

Wanted to post this for discussion/review before the 0.44 string freeze hits.

* Empty pane message:
    - say "pane" instead of "view"
    - say "open" instead of "select" (to reduce the confusion in #9254)
* JS File Inference Problem dialog:
    - ask users to file a Brackets bug
    - fix capitalization
    - format more like other errors that call out a filename
* Remove unused UNSUPPORTED_FILETYPE error & its corresponding string (I think this was leftover from an earlier part of splitview)
Ooh, good catch.  The "a" beforehand is a problem though... looks like we'll need to rephrase this a bit.  I'll put up a PR for next cycle.