Rename the unused AugurException as AugurError, to be consistent with NextstrainCliError: https://github.com/nextstrain/cli/blob/8ec779b489254c3abb7bbbf4a8f3694c4ade854e/nextstrain/cli/errors.py#L7

Previously, augur parse would let duplicates pass silently with the following behavior:

- sequence output contains all duplicates
- metadata output contains only the last occurrence of duplicates

The behavior is unintentional and undocumented.

This small change will immediately raise an error and exit when a duplicate is detected.

This isn't used for anything now, but would make things easier if there is any need to add properties/methods to all custom exceptions in the project.

print(file=sys.stderr) is used a lot in the codebase. This new function print_err acts as an alias to the original call.

Previously, augur filter would let fail on duplicates with a cryptic error:

    AttributeError: 'DataFrame' object has no attribute 'name'

This small change will immediately raise a more meaningful error and properly exit when a duplicate is detected.