Clarify unknown NPC error in mapgen

This error occurs when the game is supplied an invalid npc_template for
`place_npcs` in a mapgen entry. However, it says it was supplied an
invalid npc class, which is a separate type, leading to confusion.
Also, stick the thing it's having a problem with there, just for
redundancy.

src/mapgen.cpp

@@ -876,7 +876,8 @@ class jmapgen_npc : public jmapgen_piece
             npc_class( jsi.get_string( "class" ) )
             , target( jsi.get_bool( "target", false ) ) {
             if( !npc_class.is_valid() ) {
-                set_mapgen_defer( jsi, "class", "unknown npc class" );
+                set_mapgen_defer( jsi, "class", string_format( "unknown npc template %s",
+                                  jsi.get_string( "class" ) ) );
             }
             if( jsi.has_string( "add_trait" ) ) {
                 std::string new_trait = jsi.get_string( "add_trait" );