If ToObject() has thrown, do not throw another exception. The reason
this does not currently fail is that 1. Errors used to be created
through JS natives, and 2. the JSEntryStub clears any pending
exceptions. So, when calling into JS to create the new error, the old
exception was cleared.

BUG=5259

Review-Url: https://codereview.chromium.org/2208683002
Cr-Commit-Position: refs/heads/master@{#38300}