libceph: don't overwrite specific con error msgs

libceph: don't overwrite specific con error msgs

- specific con->error_msg messages (e.g. "protocol version mismatch")
  end up getting overwritten by a catch-all "socket error on read
  / write", introduced in commit 3a140a0d5c4b ("libceph: report socket
  read/write error message")
- "bad message sequence # for incoming message" loses to "bad crc" due
  to the fact that -EBADMSG is used for both

Fix it, and tidy up con->error_msg assignments and pr_errs while at it.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>