Add more information to 'An error occurred.' messages

There are eleven different places in the ajax processing where errors give you nothing but a cryptic 'An error occurred.' message. Since there are so many of them, you don't even really have a starting place to know what to look for. After tearing my hair out trying to debug these errors with no information, I finally realized there is a fairly simple way to provide useful feedback -- just display the url that produced the error so you can try it manually to see exactly what error was produced.

Patch attached in case that sounds useful.