Emails are made unique. When user attempts to sign in through a provider in which his email is one that is already registered, user is redirected to the signin page with an error passed as a query string parameter.
- `/{api|modules|lib}/*` returns error page when path doesn’t exist
(from Express).
- `/*` always returns index (from Express), but if `$state` doesn’t
exist, Angular redirects to `/not-found` (no 404 status in that case
though!)
- If `Accept: application/json` header is present without `Accept:
text/html`, return error as json. Hence looking at non existing /api/*
paths with browser would show html error, but querying them with script
would return json.
- Slightly prettier 404 error
Test:
```bash
curl http://localhost:3000/api/notfound -4 -H "Accept: application/json"
```
=> json error.
```bash
curl http://localhost:3000/api/notfound -4 -H "Accept:
text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0
.8"
```
=> html error (imitates Chrome’s Accept header).
Starting point was @dotch’s PL: https://github.com/meanjs/mean/pull/503
And `req.accepts()` idea came from http://stackoverflow.com/a/9802006
