Fix import and agendaupdate to handle encoding errors more gracefully

[dbarnett]

Configures file arguments with errors='replace' so that if they encounter characters they can't encode/decode they'll just replace them with a replacement marker such as '?' instead of raising an exception.

Fixes #387 (probably)

[dbarnett]

This should help fail more gracefully and I can't think of any particular downside, besides maybe that it's a bit too silent if it hits an encoding/decoding issue.

If people still run into these kinds of issues, some other ideas to try:

• Assume utf-8 encoding instead of platform-dependent?
• Dynamically retry with another encoding if the default fails and utf-8 or some other encoding might work?

But it would be good to flush out the queue of very old reports, get a conservative fix out there, and understand what issues might still remain (and what encodings might actually be involved in practice in those cases).