"can't find a pluggable transport proxy" errors when obfsproxy started up

brade in #10418 (moved) gave some reproducible instructions on how to trigger the "can't find a pluggable transport proxy" error, even when obfsproxy has no trouble starting up.

You can find brade's post in comment:40:ticket:10418

We were supposed to connect to bridge '1.3.4.5:12345' using pluggable transport 'obfs3', but we can't find a pluggable transport proxy supporting 'obfs3'. This can happen if you haven't provided a ClientTransportPlugin line, or if your pluggable transport proxy stopped running.