augur curate could have a more helpful error message

[genehack]

Context

When #1530 popped up, it was from a large "curate" pipeline where the parse-genbank-location invocation was in the middle of the stack — and because the error was happening during the output validation, I initially inferred that the problem was in a latter step because I could see the command running and seeming to finish.

This would have been much easier to figure out if the error message included the sub-command name. Currently that message is:

            Something unexpected happened during the augur curate command.
            To report this, please open a new issue including the original command:
                <https://github.com/nextstrain/augur/issues/new/choose>

Description

Add the subcommand name to the error message