Clear error message in case of invalid file

[barsan-md]

Hi,
This is a great extension but the error cases could be better handled.
Suppose I have a bad html/twig file:
<div>Ok</a>
If you try to format this document, nothing happens. If you open output and select "Prettier" from the dropdown menu, then you see "an error occurred".

A better move could be improve that "an error occurred" message with something like "There is a syntax error, the document could not be formatted." and show an informative window like the usual ones from Visual Studio Code.