Refuse malformed requests and report why they are invalid #76

Background

A request can be refused for a size below one, or for an area needing more tiles than the store can hold.

Description

Decide how a refusal is reported, and structure errors the same across both the cli and web endpoint.

State what was wrong with the request and log it.

The web endpoint keeps running after a refusal and returns it in a form the browser can show someone.

A refused request leaves nothing part generated behind.