If accounts.conf contains an invalid maildir url, return a nice
error instead of panicking.
Log a couple of different error cases to provide extra
information about the error to the user.
Add the initial implementation of a backend for Maildir accounts. Much
of the functionality required is implemented in the go-message and
go-maildir libraries, so we use them as much as possible.
The maildir worker hooks into a new maildir:// URL scheme in the
accounts.conf file which points to a container of several maildir
directories. From there, the OpenDirectory, FetchDirectoryContents, etc
messages work on subdirectories. This is implemented as a Container
struct which handles mapping between the symbolic email folder names and
UIDs to the concrete directories and file names.
