throw an exception when trying to push a background job that wont fit in the table

otherwise it will lead to failures in json decode and unexpected and hard to debug issues when running the job

lib/private/backgroundjob/joblist.php

@@ -57,6 +57,9 @@ class JobList implements IJobList {
 				$class = $job;
 			}
 			$argument = json_encode($argument);
+			if (strlen($argument) > 2048) {
+				throw new \InvalidArgumentException('Background job arguments cant exceed 2048 characters (json encoded');
+			}
 			$query = $this->conn->prepare('INSERT INTO `*PREFIX*jobs`(`class`, `argument`, `last_run`) VALUES(?, ?, 0)');
 			$query->execute(array($class, $argument));
 		}