Log request headers, response headers and some body as warning on 500s

[jml]

Re-do of #41 which was reverted in #56 because I thought it introduced an error that stopped the users service from working. Actually, it was because of #47, which we've fixed with weaveworks/service#1374

This is mostly the same as #41, but

• only logs 500s
• fixes a logic bug such that we don't try to log nil headers
• re-introduces a StatusOK thingummy, so we don't log huge amounts of information on successful requests