Do not check for a valid address, rely on client.

An email address regexp that supports all (or almost) emails is really
complex, and is_address uses a simpler one since we restrict the valid
usernames and provider names.
For that we rely on the email client to check whether the email address
is valid or not.
Remove '+' suffix in email address.

This allows us to support encrypt mails to 'test_user@provider.com'
even if was sent to 'test_user+optional_string@provider.com'.
Add changes file for #4491.
Merge remote-tracking branch 'ivan/bug/4491_remove-failing-regex' into develop
Fix error returns on openpgp backend.

Also fix:
  - some docstrings.
  - variabled referenced without previous assignement.
Merge remote-tracking branch 'drebs/bug/fix-error-returns' into develop
Improve exceptions names and handling.
Merge remote-tracking branch 'drebs/feature/improve-exceptions-names-and-handling' into develop
Fold in changes
Merge branch 'release-0.3.7'