Added examples, tweaked print job

src/PrintJob.php

@@ -94,6 +94,12 @@ class PrintJob
      */
     public function submit()
     {
+        if (!file_exists($this->document)) {
+            throw new LprException(
+                sprintf("The document could not be printed because the file could not be found: %s", $this->document)
+            );
+        }
+
         $options = [];
         if ($this->server !== null) {
             $options[] = "-H";