Updated error message when support move is used instead of convoy.

Improved error message for the situation described [here](http://webdiplomacy.net/forum.php?viewthread=1148860). This assumes that an invalid fromTerrID is due to the support move instead of convoy situation.

Note: There's no entry in the translation tables for the old error message, so I didn't include an updated translation for this new message (also, I don't speak Italian).