ob-plantuml: raises an error when the :file header argument is not set

* lisp/ob-plantuml.el (org-babel-execute:plantuml):

lisp/ob-plantuml.el

@@ -54,7 +54,8 @@
   "Execute a block of plantuml code with org-babel.
 This function is called by `org-babel-execute-src-block'."
   (let* ((result-params (split-string (or (cdr (assoc :results params)) "")))
-	 (out-file (cdr (assoc :file params)))
+	 (out-file (or (cdr (assoc :file params))
+		       (error "plantuml requires a \":file\" header argument")))
 	 (cmdline (cdr (assoc :cmdline params)))
 	 (in-file (org-babel-temp-file "plantuml-"))
 	 (cmd (if (not org-plantuml-jar-path)