A new middleware aborts any requests which require the data cache if the
cache is not yet initialized. An explicit error is returned so the admin
will be immediately aware what has gone wrong.
Previously, webpages were rendered and JSON responses were sent with
zero values, and with no indication of anything being wrong. This was
sometimes difficult to notice, and even when noticed the cause was not
immediately apparent.
Exporting this struct is a step towards breaking up the Start func into
a New func and Run func, enabling calling code to use:
api := webapi.New()
api.Run()
WebAPI is a more suitable name than server because it matches the
package name, and also it helps to distinguish WebAPI from the HTTP
server it uses internally (ie. webapi.server rather than server.server).
