parse.c: don't print error message on callback rv > 0

A callback return value < 0 means there was an error, so print
out an error message.  But a rv > 0 is used by the mount_unknown_fs
functions to say "we found the one we want, stop here."

Document this, and only print an error message if rv < 0.  Otherwise,

	lxc-create -B lvm --fstype ext3 -t ubuntu -n u1

will print an (innocuous) error message about being unable to parse
the config value 'ext3'.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>

src/lxc/parse.c

@@ -102,7 +102,10 @@ int lxc_file_for_each_line(const char *file, lxc_file_cb callback, void *data)
 	while (getline(&line, &len, f) != -1) {
 		err = callback(line, data);
 		if (err) {
-			ERROR("Failed to parse config: %s", line);
+			// callback rv > 0 means stop here
+			// callback rv < 0 means error
+			if (err < 0)
+				ERROR("Failed to parse config: %s", line);
 			break;
 		}
 	}