errorDescriptionForValue() should not assume error value is an Object

errorDescriptionForValue() should not assume error value is an Object
https://bugs.webkit.org/show_bug.cgi?id=119812

Patch by Chris Curtis <chris_curtis@apple.com> on 2013-08-22
Reviewed by Geoffrey Garen.

Added a check to make sure that the JSValue was an object before casting it as an object. Also, in case the parameterized JSValue
has no type, the function now returns the empty string.
* runtime/ExceptionHelpers.cpp:
(JSC::errorDescriptionForValue):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@154457 268f45cc-cd09-0410-ab3c-d52691b4dbfc