tombstoned: log where we're writing the tombstone.

Make it easy to find out where a specific crash's tombstone was written
to by adding a log.

Bug: http://b/62268830
Test: crasher
Merged-In: I1961dfb19f76a42a8448ebafd4be153b73cb6800
Change-Id: I7da49a96bf76dbaa3558ba64539f33283f916d8f
(cherry picked from commit cb68a031)