CLC-3287-ignore-finaid-docs-with-w-status

https://jira.ets.berkeley.edu/jira/browse/CLC-3287

A Document Status of W is for Waived documents. Waived documents no longer require student action, and should be ignored.

Although these messages were never displayed, they were being logged as errors.

Now, they are expected, but ignored, and logged as a warning message.

Notes:
- Include the "W" in an array of statuses to be ignored in the future.  Right now, "W" is the only one we ignore.
- Added test to confirm that decode_status returns nil for W