Hashed Aggregator panics when aggregate fn returns an error

**Describe the bug**
A clear and concise description of what the bug is.

The hash aggregator panics when any of the aggregators returns an error.
https://github.com/apache/arrow-datafusion/blob/master/datafusion/core/src/physical_plan/aggregates/hash.rs#L443

**To Reproduce**
Steps to reproduce the behavior:

Try to calculate the variance of an array of length 1. The current implementation of variance will return an `Err` which will cause the linked code above to panic.

**Expected behavior**
A clear and concise description of what you expected to happen.

Errors should be returned, they should not be unwrapped and cause a panic.

**Additional context**
Add any other context about the problem here.

Start in datafusion/core/src/physical_plan/aggregates/hash.rs at the linked line where the aggregate result is handled. Reproduce the issue by calculating variance for an array of length 1, whose aggregate returns an error. Done means the error is returned to the caller instead of causing a panic.

Written by the indexing model from the issue text.