[EchoClient] improve unexpected echo output

Build up output string and don't incrementally output it to stderr
to hopefully prevent error messages interleaving.
Only output the least significant byte of the unsigned int cast.
Always use two digits per byte and remove the whitespace.
Should have used printf in the first place.
21 jobs for echo_client_verbose_echo_failure2 in 8 minutes and 44 seconds (queued for 2 seconds)