This tweaks the signature of `request_builder_hook` in order to support
fallible hooks.
The rationale for this is mostly on two sides:
* it allows a hook to inspect and possibly drop an outgoing request
(e.g. for policying purposes), bubbling up a detailed error message to
the user.
* it wires into newer `reqwest` API which allows to split and then
reassemble a `RequestBuilder`, although only in a fallible way
(https://github.com/seanmonstar/reqwest/pull/1770)
3f26ee8604