Do not cause an error when trying to stringify a NativeError.

[gbrail]

This is currently the bug:

$ rhino
Rhino 1.7.6 2015 04 15
js> try { somethingundefined(); } catch (e) { JSON.stringify(e); }
js: Java class "[B" has no public instance field or method named "toJSON".

With the fix, the proper behavior is this:

Rhino 1.7.7 2015 06 17
js> try { somethingundefined(); } catch (e) { JSON.stringify(e); }
{"message":""somethingundefined" is not defined.","fileName":"","lineNumber":2}