Only log an error if an unattached thread is unregistered.
Only log an error if an unattached thread is unregistered.

Turn a fatal check into a diagnostic error. It looks like this issue arises
when a runtime shutsdown without properly being started.
Bug: 17011539

Change-Id: I2983c8332e83769e9480e8f30a46ca3b80a2e90e

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst