org-src.el (org-edit-src-code): Throw a warning instead of an error

* org-src.el (org-edit-src-code): Throw a warning instead of
an error when loading the mode fails, otherwise the user is
left with unusable buffers.

Thanks to Florian Beck for suggesting this.

lisp/org-src.el

@@ -348,7 +348,7 @@ the display of windows containing the Org buffer and the code buffer."
 	  (condition-case e
 	      (funcall lang-f)
 	    (error
-	     (error "Language mode `%s' fails with: %S" lang-f (nth 1 e)))))
+	     (message "Language mode `%s' fails with: %S" lang-f (nth 1 e)))))
 	(dolist (pair transmitted-variables)
 	  (org-set-local (car pair) (cadr pair)))
 	;; Remove protecting commas from visible part of buffer.