Refactor kv::format_error to cf::format_error

[EverlastingBugstopper]

Any API call we make with cloudflare-rs should be formatted in a single function. Looking at you https://github.com/cloudflare/wrangler/blob/master/src/commands/kv/namespace/site.rs#L42

This is related to #360

[gabbifish]

The kv::format_error function calls upon the help() function to print out helpful details when the API returns an error. Currently, this function looks for KV-specific error codes to figure out which help message to print. So, if we generalize this help function to cf::format_error, we need to extract this help() function and ideally have it provide helpful messaging for other CF API error codes.

[ashleymichal]

we may need to define multiple help functions for different apis, as there will be collisions in codes between them, e.g.
workers api error codes:
https://api.cloudflare.com/#worker-script-errors
workers kv api error codes:
https://api.cloudflare.com/#workers-kv-namespace-errors

[gabbifish]

#841 actually already fixed this: it introduces a http::format_error function that takes a callback as an argument. This callback function takes an error code and returns a string providing more information about it. This means we now have a general error formatting function for the CF API's error responses, which any subcommand that calls a particular API (e.g. the routes API) can use to format CF API errors.