File exists check

[OwenD]

It would be good if we could get the result of whether a file exists in G Code.
For example could M36 be changed to return an error code 1 or 2 if the file isn't found?
At present it returns the error state as part of the JSON response but returns and error state of zero even if the file isn't found.

[dc42]

I've added this to the work list.