Give a more specific error message in cases where Brackets refuses to open
a file that is not UTF-8 (message is also shown when trying to open binary
files).

The right error code is already returned from the native shell -- we just
need to plumb it down to FileSystem and add a case-specific string for it.
Merge pull request #7787 from adobe/pflynn/encoding-error-string

More specific error message when non-UTF-8 files opened