Should `req_error()` have more control?

[hadley]

i.e. should we give the user the ability to throw their own error? Related to #196

[jonthegeek]

I feel like I want the ability to do this as long as it doesn't make things harder to use.

At first it was just "this feels useful," but I can definitely imagine cases where I'd want to pass along argument names and (high-level) calls per error chains. In fact I suspect I'm a day or two away from requesting that specifically.