Enh: import-mbox needs error handling to report the context of any errors

If a message causes a problem (e.g. due to an encoding issue), a stack trace is not much use without additional details, for example the message being processed.

Start at the import-mbox entry point and trace how messages are processed and how encoding or other errors are currently reported. Define the error context from the issue, then verify that failures identify the message being processed rather than only emitting a stack trace.

Written by the indexing model from the issue text.