Change wtf errors from ASSERT to ERROR.

Calling println_native with the value of ASSERT will set the abort
message. Unfortunately, there are numerous cases where one thread aborts,
but before that thread can communicate with debuggerd, another thread
calls wtf, which overwrites the abort message from the original
failing thread.

Fix this by changing this to an error level log message.

Bug: 21490722
(cherry picked from commit 2d073ba8)

Change-Id: Ib9daedf114a2e0ecc424641e4929cc4d15b0c776