Change error message with g_key_file_get_string

Current error message(G_KEY_FILE_ERROR_GROUP_NOT_FOUND) is

~~~
[root@localhost ostree]# ostree remote show-url bar
error: Key file does not have group 'remote "bar"'
~~~

This should be clear. And this patch will change the message as below
~~~
[root@localhost ostree]# ./ostree remote show-url bar
error: url not found in 'remote "bar"'
[root@localhost ostree]# ./ostree config get 'remote "bar"'.url
error: url not found in 'remote "bar"'
~~~