CLI: exit with error when load_config returns an error.

For now print a generic error message and exit with error on any
non-success code. Previously the code exited, but with exit code zero,
leading users / scripts to think the command had succeeded.

notmuch.c

@@ -584,6 +584,8 @@ main (int argc, char *argv[])
 	case NOTMUCH_STATUS_SUCCESS:
 	    break;
 	default:
+	    fputs ("Error: unable to load config file.\n", stderr);
+	    ret = 1;
 	    goto DONE;
 	}
 

test/T030-config.sh

@@ -196,7 +196,6 @@ output=$(notmuch config get built_with.nonexistent)
 test_expect_equal "$output" "false"
 
 test_begin_subtest "Bad utf8 reported as error"
-test_subtest_known_broken
 cp initial-config bad-config
 printf '[query]\nq3=from:\xff\n' >>bad-config
 test_expect_code 1 "notmuch --config=./bad-config config list"