handle read-only mailboxes gracefully
As mentioned over in #111, I have some configurations where i want to use geary to access a read-only, receive-only mailbox. (e.g., generating screenshots for how Geary renders certain messages, like imap://bob@protected-headers.cmrg.net, see https://header-protection.cmrg.net/), So attempts to do things like create special folders will always fail and cause geary to show some fairly scary/incomprehensible warnings to the user too.
I grant that this use case is relatively obscure, and maybe whoever is testing messages in this context just needs to work around these warnings. But it does increase the burden of generating screenshots from a public corpus if there are a lot of hoops like this to jump through for a new setup.