Bug 960749 - Improve error messages when < and > are used in filenames. r=bsmedberg

* Unify OS_FILE_ILLEGAL_CHARACTERS
* Provide KNOWN_PATH_SEPARATORS in addition to FILE_PATH_SEPARATOR.
* Update nsExternalAppHandler to use KNOWN_PATH_SEPARATORS instead of
  FILE_PATH_SEPARATOR when removing path separators.
* Fix DownloadsIPC.jsm when compiling with MOZ_DEBUG (would throw on undefined download.id).
* Add test for stripping out illegal characters when downloading.