The error message without any context/line number information.
@note nsIConsoleMessage.message will return the error formatted
with file/line information.
Categories I know about -
XUL javascript
content javascript (both of these from nsDocShell, currently)
system javascript (errors in JS components and other system JS)
pseudo-flag for default case */
message is warning */
exception was thrown for this case - exception-aware hosts can ignore */
error or warning is due to strict option */