BLUEGENE - Change error to debug, sense the issue really isn't an error,

more of a hey, the user is asking for something out of the norm.

src/plugins/select/bluegene/bg_dynamic_block.c

@@ -250,7 +250,7 @@ try_small_again:
 			break;
 #endif
 		default:
-			error("This size %d is unknown on this system", cnodes);
+			debug("This size %d is unknown on this system", cnodes);
 			goto finished;
 			break;
 		}