fix possible issue regarding bytes/str decoding

[LeiGlobus]

This started giving the following mypy error recently. Not quite sure what changed, but handling str vs. bytes formally is better anyway. (line a bit long for a one-liner so split it into if/else)

globus_compute_endpoint/engines/high_throughput/messages.py:110: error: If x = b'abc' then f"{x}" or "{}".format(x) produces "b'abc'", not "abc". If this is desired behavior, use f"{x!r}" or "{!r}".format(x). Otherwise, decode the bytes  [str-bytes-safe]