Improve error message for shifting elements back in time

When an element is shifted forwards in time (often to the end of the Global Window) as a result of a GroupByKey, they can't be reassigned the timestamps the user expects. This is fine, but users don't understand why the elements have been shifted forwards, and we should improve the messaging around this error.

Imported from Jira [BEAM-2100](https://issues.apache.org/jira/browse/BEAM-2100). Original Jira may contain additional context.
Reported by: tgroh.

Start by locating the GroupByKey path that shifts elements forwards in time, particularly when they reach the Global Window, and inspect the error message it produces. Review nearby tests or error-handling code to understand the current behavior. Done means the error clearly explains why timestamps were shifted and why the user cannot reassign the expected timestamps.

Written by the indexing model from the issue text.