Support error handling

[multimeric]

One feature that I find myself often wanting in bench is the ability to run benchmarks that may fail.

It's actually not trivial to implement this in the current framework, because handling errors would involve returning the error objects which would fail the equality check. Then if you ever set check=FALSE then results are not captured at all.

What I would love to see is automatic error handling, whereby any error is caught by bench, and added to a new column called errors, which might be a list of error objects, which are NULL if the expression succeeded. Then the result column can work as normal.