Modify missing output fd message.

The "missing output fd" message can seem like an error, so modify
the message to indicate what is really happening. This message
will occur normally when running the debuggerd command, or when
a bugreport is generated, or when an ANR occurs. In all of those
cases, this is not an error, but an expected action.

Bug: 196189981

Test: Ran debuggerd -b and debuggerd and verified this message is seen.
Test: Ran unit tests.
Change-Id: I6e3d5a76d92b972c77fca301ea7147745bc67c37