JDWP: avoid crash on unsupported modifier

Returns NOT_IMPLEMENTED error if we receive an event request with an
unsupported modifier.

Bug: https://code.google.com/p/android/issues/detail?id=81037
Bug: 18617787
(cherry picked from commit 0fb33e7b7b0b86fb9c7b556e4a052874ec269bb0)
Change-Id: I97729e2f98af3a75d24604926a89860255d4acae