Return AggregateExpressionError from aggregates stage (#843)

<!-- The PR description should answer 2 (maybe 3) important questions:
-->

### What

This stage already has it's own error type, but it returns the larger
`Error` type, so let's return the more specific type instead.

<!-- What is this PR trying to accomplish (and why, if it's not
obvious)? -->

<!-- Consider: do we need to add a changelog entry? -->

### How

Changing return types mostly. Functional no-op.

<!-- How is it trying to accomplish it (what are the implementation
steps)? -->

V3_GIT_ORIGIN_REV_ID: 2aae1f06775db6d88c34b1d3c1779396e0ba410e