[execution][messagehandlers] Do not systematically decode incoming zmq parts

Under some circumstances the decoding may fail as the binary
data may contain values outside the utf-8 "visible" range.

Fixes #26
4 jobs for 26_fix_decoding_error in 8 minutes and 26 seconds (queued for 7 seconds)
Status Name Job ID Coverage
  Build
passed build_linux_36 #174326
docker

00:08:22

73.0%
passed build_linux_37 #174327
docker

00:06:46

73.0%
passed build_macosx_36 #174324
macosx

00:04:00

73.0%
passed build_macosx_37 #174325
macosx

00:03:59

73.0%