fix(services/version): prevent checkForUpdate() 5xx response codes from stopping connection

[crookse]

I was receiving this error earlier today and couldn't connect to my process:

I found the issue was around the response code returned from fetching from UPDATE_URL (value is https://get_ao.g8way.io) for an update. The response code was 502.

Although this PR doesn't address every edge case, it addresses the one I experienced earlier today. This PR introduces the following:

• Returns { available: false } if fetching from update URL returns a 5xx status code
• Adds tests to ensure 404 and 5xx status codes return { available: false }
• When process.env.DEBUG is truthy, a warning message will be shown for fetches that result in 5xx status codes. For example:
  

[twilson63]

LGTM