The commentary is not yet complete. It currently stops at chapter 4 of
the RFC.

RFC 3588 allowed only 3xxx result codes in an answer-message (that is,
an answer that sets the E-bit) while RFC 6733 also allows 5xxx result
codes. Setting request_errors = answer tells diameter to answer 5xxx
errors itself. Returning {answer_message, integer()} from a
handle_request callback allows both 3xxx and 5xxx result codes to be
set. {protocol_error, integer()} is retained for 3xxx result codes.

When receiving a request for which errors have been detected during
decode, diameter previously used the errors list in the decoded
diameter_packet record to unconditionally set Result-Code and Failed-AVP
in the outgoing answer. It wasn't particularly delicate in doing so
however and would happily set a 5xxx Result-Code even if a
handle_request callback returned an answer-message, leading to an encode
error. This behaviour became even less endearing as of commit ac452e28,
which made it possible to handle_request to take place even for protocol
errors. (ie. When a callback typically should return an answer-message.)

This commit fixes the behaviour by only setting a value that's
appropriate for the answer in question, either a 3xxx or a 5xxx,
depending on if the answer's an answer-message or not. It also allows
handle_request to prevent diameter from setting anything by setting
errors = false in a returned diameter_packet. Ideally it should have
been errors = [] but the empty list is the default value for the errors
field and changing the default (ideally there shouldn't have been one)
would require recompilation of all modules including diameter.hrl:
choose the less attractive 'false' to avoid such backwards
incompatibility.

The request reception is also refactored somewhat to shorten some call
chains.

Configuring the value 'callback' all errors detected in incoming
requests to result in a handle_request callback. The default value
'answer_3xxx' is the previous behaviour in which diameter answers
protocol errors without a callback.

The value determines whether or not an unexpected message length in the
header of an incoming messages causes the peer process to exit, the
message to be discarded or handled as usual. The latter may only be
appropriate for message-oriented transport (eg. SCTP) since
stream-oriented transport (eg. TCP) may not be able to recover the
message boundary once a length error has occurred.

Don't start a new timer with each incoming message. Instead, start a
timer at timeout and flush after two successive timeouts with no message
reception.