(bug 11142) Improve file ext error reporting in API upload

(bug 11142) Improve file ext error reporting in API upload

The API action=upload returns a generic error message when one or more
of the file extensions are blacklisted. It should return a more
informative message, and also return the list of blacklisted extensions
in a machine-readable format.

This changeset introduces a new message,
"api-error-filetype-banned-type", which is to
"api-error-filetype-banned" what "filetype-banned-type" is to
"filetype-banned". As a starting point, I've copied (with minor changes)
the existing filetype-banned-type message to
api-error-filetype-banned-type for all languages for which both
api-error-filetype-banned and filetype-banned-type are currently given.

This changeset also incidentally fixes an internal server error with
format=xml when the filetype-banned error is returned by the API.

Change-Id: I6b0c58fbc5b19aa55286c56fa7da2195ea683ae0