Correctly send MethodExit events from exception handlers

Due to the way that exceptions are handled it was possible for a
MethodExit event to be sent multiple times for the same exception.
This fixes that issue by correctly keeping track of which exception
events have already been sent and correctly tracking the current
exception being thrown.

Test: ./test/testrunner/testrunner.py --host --runtime-option=-Xplugin:libtracefast-trampolined.so
Test: ./test.py --host

Change-Id: I7fa09f0f3f82181430b18805da5ba8daf68d4b89