[Evolution] Interaction with ProtonMail “Error syncing changes: mailbox 4 does not exist”

[Dan Kortschak <dan kortschak io>]

Sporadically when I attempt to empty the trash on a ProtonMail account
using evolution, I get the following error:

```
Failed to refresh folder "<user>@protonmail.com : Trash”.

The reported error was “Error syncing changes: mailbox 4 does not
exist”.
```

then when navigating away from the trash folder, 

```
Error while Storing folder “Trash”.

Error syncing changes: mailbox 4 does not exist
```

> From this point on, evolution will not sync to the server (see note
below).

This problem is only fixed by quiting evolution and killing evolution-
source-registry, and going to the ProtonMail web client and emptying
the trash.

Note that ProtonMail is odd in this since it doesn't provide IMAP
access to their servers, but rather has a bridge application that
interacts with their servers and presents an IMAP server on localhost.

I intend to instrument the ProtonMail bridge application, but I would
like to have a basic idea of what it is that I should be looking for
before I jump into that.

```
~ $ evolution --version
evolution 3.28.5-0ubuntu0.18.04.2 
```

thanks
Dan Kortschak