fix/login: validate keyring oauth creds and re-auth if needed

[burmudar]

If we have loaded from the OAuth store and validation fails with the token, re-launch the OAUth flow

Closes PLAT-701

Test plan

1. src login https://sourcegraph.test:3443
2. revoke in UI
3. src login https://sourcegraph.test:3443

./src2 login https://sourcegraph.test:3443
The OAuth token is invalid. Please check that the Sourcegraph CLI client is still authorized.

To re-authorize, run: src login https://sourcegraph.test:3443

Learn more at https://github.com/sourcegraph/src-cli#readme


⚠️  Warning: Stored OAuth credentials could not be verified. Starting a new OAuth device flow.

If your browser did not open automatically, visit https://sourcegraph.test:3443/auth/device?user_code=JNELRRVD.
Waiting for authorization... DONE


✔︎ Authenticated as sourcegraph on https://sourcegraph.test:3443


✔︎ Authenticated with OAuth credentials