When something goes wrong with the imaplib.IMAP4_SSL connection (like the host is
temporarely down or the DNS does not resolve) it generates an OSError which is currently
not catched and handled. Now OSErrors are translated to MailFetcherErrors which get
logged and the IMAP connection is retried in the next IMAP check.
Fixes#474
Make the sleep duration dynamic to account for the time spent in
loop_step.
This improves responsiveness when repeatedly consuming newly
arriving docs.
Use float epoch seconds (time.time()) as the time type for
MailFetcher.last_checked to allow for natural time arithmetic.
Check list length before accessing the first two elements of
'dispositions'.
The list may have only a single element ('inline') or may be empty in
mailformed emails.
Previously, all errors raised during mail fetching were silently caught
and printed without backtrace.
To increase robustness and ease debugging, we now fail with a backtrace
on unexpected errors.
Originally we used SHARED secret both for email and for the API. That
was a bad idea, and now that we're only using this value for one case,
I've renamed it to reflect its actual use.
