This commit changes MsgBlock to enforce a 1MB max payload per the spec.
Previously it was only limited to the max overall message size. While
here, also enforce max payloads per message type (instead of only the max
overall message payload) when writing messages.
The io.Writer.Write function always returns an error if the bytes written
is less than provided, so there is no reason to further check if the
payload length matches after a successful write.
This commit adds a new MessageError type that is intended to allow the
caller to differentiate between general io errors and and errors that
resulted from malformed messages.
The io.ReadFull function always returns an error if the full payload can't
be read, so there is no reason to further check if the payload length
matches after a successful read.
