Rename instrumentation ExceptionCaught to ExceptionThrown

The instrumentation ExceptionCaught event was badly named since it is
actually sent at the time the exception is thrown and is sent even if
the exception is never actually caught. Rename it and associated
methods/fields to ExceptionThrown to more accurately reflect the
event's semantics.

Test: ./test.py --host -j50
Change-Id: I1e9c401ab619259b25928dbf19f62d3fa9573d47