Fix JNI error in exception reporting.

There was a JNI error where when you got an OOM and called
report_exception, it would call two NewStringUTF in a row without
checking the return values. This could mean that the first one
threw a new OOME and the second one would cause a JNI error when
it also attempted to throw an OOME with a pending OOME.

Bug: 16843627

(cherry picked from commit cf6775ee)

Change-Id: Ibdc7d0e55a48b2a61a1db0868a5d77c2ae53f6f3