It came up when fixing bug 19907 that we don't propagate any exception information in an NPAPI callback. The NPAPI reference suggests that we should return false, at least in some cases:
Some compatibility testing will be required, because the NPAPI documentation isn't very specific.
Related to Bug 18234?
(In reply to comment #1)
> Related to Bug 18234?
Yes, that bug should probably depend on this one, or maybe even be marked a duplicate. There is now no real support for NPN_SetException.