fix access denied fault error writing js/positions.js

[sdetweil]

if the MagicMirror js folder is not writable (synology nas created by different user than docker container) there is an uncaught throw

[ERROR] EACCES: permission denied, open 'js/positions.js' 

add try/catch, output new message, console.error

unable to write js/positions.js with the discovered module positions
make the MagicMirror/js folder writeable by the user starting MagicMirror 

MM will start, but no modules will show, as no positions were discovered
this file is used to pass the list from the server side to the browser side

fixes #3651