Returns the errorOutput incrementally.
In comparison with the getErrorOutput method which always return the whole error output, this one returns the new error output since the last call.
string The process error output since the last call
public function getIncrementalErrorOutput() {
$data = $this
->getErrorOutput();
$latest = substr($data, $this->incrementalErrorOutputOffset);
$this->incrementalErrorOutputOffset = strlen($data);
return $latest;
}