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
Change-Id: Ie4f9f9a5f8b7993cd3655d42a6718c0a5e1199f8