wireprotov2peer: properly format errors formatrichmessage() expects an iterable containing dicts with well-defined keys. We were passing in something else. This caused an exception. Change the code to call formatrichmessage() with the proper argument. And add a TODO to potentially emit the proper data structure from the server in the first place. Differential Revision: https://phab.mercurial-scm.org/D4441

local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed