Be less brutal in setting Result-Code/Failed-AVP

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.

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