Handle unauthenticated dev users with grace

[RaeesBhatti]

- Do you want to request a feature or report a bug?
Feature

- What is the current behavior?
netlify dev dies miserably if you're not logged in.

◈ Netlify Dev ◈
{ FetchError: invalid json response body at https://api.netlify.com/api/v1/sites/761206b6-2410-4027-ab0b-10b8730b356d/service-instances reason: Unexpected end of JSON input
    at C:\Users\raeesbhatti\Projects\netlify\cli\node_modules\node-fetch\lib\index.js:272:32
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:189:7)
  message: 'invalid json response body at https://api.netlify.com/api/v1/sites/761206b6-2410-4027-ab0b-10b8730b356d/service-instances reason: Unexpected end of JSON input',
  type: 'invalid-json' }
◈ Error retrieving addons data for site 761206b6-2410-4027-ab0b-10b8730b356d. Double-check your login status with 'netlify status' or contact support with details of your error.

- If the current behavior is a bug, please provide the steps to reproduce.

• netlify logout
• netlify dev in your project

- What is the expected behavior?
netlify dev runs without any errors and shows a warning for the unavailable feature because of unauthenticated status.

- Local Environment Information

────────────────────┐
 Environment Info   │
────────────────────┘

  System:
    OS: Windows 10 10.0.18363
    CPU: (16) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
  Binaries:
    Node: 8.17.0 - C:\Program Files\nodejs\node.EXE
    npm: 6.13.4 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: 44.18362.387.0

netlify-cli/2.43.0 win32-x64 node-v8.17.0

[RaeesBhatti]

Closing in favor of #174