p-vital

This PR exposes the remaining attributes of nsIScriptError (https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Reference/Interface/nsIScriptError) - most importantly, `flags` which permits distinguishing errors (technically exceptions) from warnings in the code that uses JSErrorCollector.

The current master has several build fixes, it would be great if a new release could be made with them. Thanks!