Return an error string for request format errors.
Return an error string for request format errors.

Previously we just closed the socket. I was trying to use a plain http
URL through the helper, which isn't allowed by requestOK. (This is just
a safety feature because none of our deployment is planned to use plain
http.) It took me a little while to figure out what was going wrong. Now
you will get a line in the meek-client log:

error in handling request: helper returned error: request failed validation

Yawning/meek,Yawning/meek,infinity0/meek,scucoco/meek,gorshell/forkmeek,Yawning/meek,projectarkc/meek,projectarkc/meek,projectarkc/meek,infinity0/meek,projectarkc/meek,scucoco/meek,wyuelin/mymeek,infinity0/meek,scucoco/meek,wyuelin/mymeek,infinity0/meek,arlolra/meek,gorshell/forkmeek,projectarkc/meek,gorshell/forkmeek,arlolra/meek,wyuelin/mymeek,arlolra/meek,wyuelin/mymeek,Yawning/meek,infinity0/meek,gorshell/forkmeek