In celebration of my second commit on weboob

Since no one seems to use it.
I fixed the biggest issues so it is now usable again!

While this changes constants names, I think it is important as "accused" is
complely wrong. E-mails use the term "Read-receipts" so I chose
to go with RECEIVED.

Probably the result of a bad merge

There is no reason to hide it from anyone, the GPG default is not
appropriate in our case.

stderr is shown and an exception is raised if GPG does not exit
successfully.