Improve addon download refusal messages

Changes some instances of file to addon and warns the user if they cannot join due to the server adding a folder as-if it were an addon.

Contacting the server...
NOTICE: You need additional addons to connect to this server:
 * "VL_HowToDieInTwoSeconds-v1" (folder)
One or more addons was added as a folder, which the server cannot send.