Merge branch 'sprite-loading-tweaks' into 'master'

Fix R_AddSingleSpriteDef's I_Error messages

Whoops, seems I forgot about this little branch. Basically this fixes how for a character's sprites, a full sprite lump name is displayed instead of just its sprite prefix in the "R_AddSingleSpriteDef: No patches found for [sprite prefix] frame [frame character]" message (unlike when it occurs for non-character sprites), resulting in something like "R_AddSingleSpriteDef: No patches found for PLAYC2C8 frame \^" which does NOT help custom character authors at all as it just confuses them instead. (for the record, the problem would actually with the ^ frame and not the ones displayed after "PLAY")

Oh, and the same problem is also fixed for this similar message: "R_AddSingleSpriteDef: Sprite [sprite prefix] frame [frame character] is missing rotations"

See merge request !95