Errors in Diags::set_std_output() ignored, which can result in a crash.

I ran a stress test that exhausts the available file descriptors for the traffic_server process. This caused the Diags::stdout_log member of Diags to be set to nullptr by this line: https://github.com/apache/trafficserver/blob/1426a069efc54f8096159473e3e4aafc3f23a0b4/src/tscore/Diags.cc#L791 , which then resulted in a segment violation. set_std_output() returns true/false to indicate whether a this or other errors occurred, but this return value is ignore. Maybe the errors in this function should simply cause a FATAL TS error