debuggerd_handler: don't assume that abort message implies fatal.

Applications can set abort messages via android_set_abort_message
without actually aborting. This leads to following non-fatal dumps
printing their output to logcat in the same format as a regular crash.

Bug: http://b/37754992
Test: debuggerd_test
Change-Id: I9c5e942984dfda36448860202b0ff1c2950bdd07