debuggerd_handler: demote abort on exec failure to log.

If a process is ptraced already, we might not be able to exec crash_dump
due to selinux. Since we can be called for non-fatal events, we
shouldn't abort in that case.

Bug: http://b/128054996
Test: treehugger
Change-Id: I1442041caa7af908df2ab87b9e010c44082e7587