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

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.

I haven't tested this on Mac but I think this will give us the same benefit there too -- it looks like the same brackets.fs-level encoding error code is returned in the right cases there already. But if not, it's not critical since Mac has always refused to open UTF-8 (whereas for Win it's a new and potentially confusing behavior).