🐛(dimail) grab duplicate displayname error

OpenXchange's primary key is display name (= first name + last name).
In absence of clear error message from dimail (yet), we catch errors 500 and
check if they're not due to the display name already existing in the context

CHANGELOG.md

@@ -23,6 +23,10 @@ and this project adheres to
 - ✨(api) update mailboxes #934
 - ✨(api) give update rights to domain viewer on own mailbox #934
 
+### Fixed
+
+- 🐛(dimail) grab duplicate displayname error #961
+
 ### Changed
 
 - 💥(sentry) remove `DJANGO_` before Sentry DSN env variable #957