use more appropriate return value in driver_t::select_box()

don't say DRV_CANCELED when it's really DRV_STORE_BAD, as apart from
being just wrong, it lead to the confusing effect of canceling a store
as the result of a supposed cancellation of the same store.

src/drv_maildir.c

@@ -1269,7 +1269,7 @@ maildir_select_box( store_t *gctx, const char *name )
 		}
 	} else {
 		if (!(ctx->path = maildir_join_path( conf, 0, name )))
-		    return DRV_CANCELED;
+		    return DRV_STORE_BAD;
 		ctx->is_inbox = 0;
 	}
 	return ctx->path ? DRV_OK : DRV_BOX_BAD;

src/sync.c

@@ -1190,7 +1190,7 @@ sync_boxes( store_t *ctx[], const char * const names[], int present[], channel_c
 	 * don't run into uninitialized variables. */
 	for (t = 0; t < 2; t++) {
 		switch (svars->drv[t]->select_box( ctx[t], svars->box_name[t] )) {
-		case DRV_CANCELED:
+		case DRV_STORE_BAD:
 			store_bad( AUX );
 			return;
 		case DRV_BOX_BAD: