fix RPC error replies

After pull #4288, RPC messages indicating errors have a Content-Length unrelated
to their actual contents, rendering bitcoin-cli and curl unable to decode the
reply.

This patch sets the Content-Length field based on the actual content returned.

Additionally, pull #4288 clobbered the error descriptions provided in
ErrorReply, which bitcoin-cli relies upon; this patch moves #4288 http-error
descriptions to an HTTPError method, allowing HTTPReply to pass content on
unchanged.