More quality of life for the pushState feature

- adds some docs at the top for apiProxy.prefix
- if users have pushState enabled but forget to include the prefix, they
will get a warning from `requiresConfig`
- customized logging output when pushState is enabled and an apiProxy
prefix is included
- factored out the common proxyError handler
- split the apiProxy concerns relating to prefixMatching into its own
function `prefixMatchingApiProxy`