Better error messages for 400, 401 and 404

[donaldpipowitch]

Do you want to request a bug?
bug

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

400 and 401 errors are reported as 404. See #2151 for an example. It took me a while to figure out that #2151 was actually an auth bug with a 401 error status code, because it was reported as a "not found" error.

I think this bug occurs because of this line.

What is the expected behavior?

Report errors like this:

• 400: error Couldn't install package "foo" on the "npm" registry, because there was an unknown problem. ❌
• 401: error Couldn't install package "foo" on the "npm" registry, because you're unauthorized. ❌
• 404: error Couldn't find package "foo" on the "npm" registry. ✅

Please mention your node.js, yarn and operating system version.

$ node -v
v6.9.1

$ yarn -V
0.18.0

macOS 10.12.1

[Daniel15]

http.STATUS_CODES can be used to get the right error message based on the code. The code that handles the error should probably be doing that.