Modified default/failed return value.

I would like to implement a usable return value for failure. The string seems fine, however a value that would always produce a valid JSON object makes life easier on the receiving end.

For instance: a case that fails can fall through ( in api.php, and `break;` on success ), or a javascript client can rely on `if( json.result ){}`

This may of course require the current api options to not touch $result on failure, which is easily implemented.

If you are interested in the idea, or something similar, I will go and verify the current API to ensure failure is correctly handled, and add the commits here.