Make PermissionDeniedError struct more detailed

[markan]

The PermissionDeniedError struct is contains only simple text string created at the time of error generation. We'd like to make that data structured to allow better user messages.

Currently we have

type PermissionDeniedError struct {
	Cause string // which might be the full error message rendered at the point of creation...
}

In many cases we have detailed information about the reason for rejection and would like to capture that.
We should extend with this info. Not all PermissionDenied errors fit that template, so we should create a new structure to capture this.

type PermissionDeniedByACLError struct {
	Accessor string // "token guid"
        Permission string // e.g. "service:read" Perhaps split into resource and level 
        Type string // e.g. service
        Name string // e.g. "sidecar-proxy-1"
        EnterpriseMeta 
}

The acl.PermissionDenied helper function usage should also be examined to see where we need to convert to the new form.

A string conversion function similar to what PermissionDeniedError has should be added.

https://github.com/hashicorp/consul-enterprise/pull/1533