Properly handle weird filenames about kamaki HOT 1 CLOSED

Implementing the Swift API means kamaki has to pass filenames as part of HTTP URLs.
To do that, it first encodes the filename string as UTF-8, then URL-encodes it, to make it part of the URL.

This means it has to know the encoding of a filename passed to it at the command line.
It assumes the encoding matches the locale settings of the user.

If your locale settings specify UTF8 as your preferred encoding, then the problem is that the filename you are specifying is not valid UTF8. If this is the case, then the error message should change to better reflect this, e.g., "The string '...' is not valid based on your locale settings (codec: 'utf8').".

from kamaki.