Allow req_perform_iterative to use the verbosity, mock, and error_call arguments from req_perform

[guslipkin]

Right now not all options from req_perform are available to req_perform_iterative. Adding verbosity especially would help in debugging.

I tried setting this up in #595.

[hadley]

Why not use req_verbosity() or with_verbosity()?

[guslipkin]

I'm using req_perform_iterative() in a package and leaving it as an argument makes it much easier to expose that option to the user.

I guess it's just a bit unclear to me why it is an option in req_perform() but not req_perform_iterative(). If req_verbosity() or with_verbosity() are an option, why does req_perform() have it as an argument at all? And if it's helpful enough that req_perform() gets it, then why shouldn't req_perform_iterative() or req_perform_parallel() or req_perform_sequential()?

[hadley]

Basically, I'm ambivalent about the verbosity argument, so I'm not sure I want to spread it to more places, given that there are alternatives.