Add error messages for failed submodule lookup

src/submodule.c

@@ -128,6 +128,10 @@ int git_submodule_lookup(
 			git_buf_free(&path);
 		}
 
+		giterr_set(GITERR_SUBMODULE, (error == GIT_ENOTFOUND) ?
+			"No submodule named '%s'" :
+			"Submodule '%s' has not been added yet", name);
+
 		return error;
 	}