Clearer interaction state algorithms

[marcoscaceres]

Elsewhere, @stephenmcgruer suggested:

we [should] capture the various repeated steps for "shutting down" payment request into their own steps somewhere, so we don't have to keep repeating and missing ones? Something like:

Steps to quit out of payment handler due to error:

• Set state to closed
• Set is-showing to false
• Queue a task to reject (relevant promise) with the passed-in error type