Improve errors when scope is limited

Problem/Motivation

If the API token only has `purge_select` and `purge_all` then, when you visit the Edge modules page, the error shows "403" which isn't obvious.

Steps to reproduce

• Create a token with limited scope.
• Set the token.
• Visit /admin/config/services/fastly/edge-modules.
• Note the generic error.

Proposed resolution

Add handling to inspect the token and offer an information message.

Remaining tasks

User interface changes

API changes

Data model changes