Fix CLI usage with no pre-set context/config

[tstirrat15]

Description

In #417 we introduced a bug where zed became unusable if there wasn't previously a context set by the user. This wasn't intentional; it should be valid to invoke zed entirely from the command line in isolation. This PR captures that behavior in a unit test and fixes the behavior.

Changes

• Separate existence and fetching concerns for tokens
• Handle cases where config file or token don't exist

  Testing

  See that tests go green. See that you can clear out your local context (or run from within a container): /zed schema write ./blah.zed --endpoint 127.0.0.1:50051 --token supersecretdevkey --insecure and not receive an error.

[tstirrat15]

Follow-on work around code organization can happen in the context of #421.