error log from exec?

I am running this line in my controller where I am converting a pdf to an png. 

exec('soffice --convert-to png {$file_uploaded} --outdir {$file_image}');  //convert to image

It is not working but I don't know why? Is there a place where error message from exec are logged? 

You can pass a 2nd and 3rd argument to the exec function. The 2nd argument must be an array and the 3rd must be an integer.

PHP Code:

$output = [];
$return = -1;
exec('soffice --convert-to-png {$file_uploaded} --outdir {$file_image}', $output, $return);

// print the output
print_r($output);

// is this an error?
echo $return; // if 1, then error; 0 if successful