Adds in additional functionality to remove instances and update instances to existing endpoints.
Also includes a bit of code cleanup and adds an accessor function to the existing slice API
(so that we can determine if a given object is a "uri_root") and onverts "error!" calls used
by earlier versions to code that raises (and handles) a ProjectRazor::Error::Slice::MethodNotAllowed
exception instead; this should make the behavior more consistent with what was seen in previous
versions of Razor (and less Rack-specific)