Error handling in ionic2/ Angular2 - php

Question

Trying to integrate APIs built in Laravel 5.4 with ionic 2 and struggling handle the error
What I want to do:
Authenticate the login using Laravel Password service ( OAuth2 ).
Once authenticated, it would return the access token.
Access Token is passed in the header in a GET API call to receive the
user details.
I am able to #1 and #2 but got stuck at #3.
Here is my code of login.ts
public login() {
this.showLoading();
this.auth.login(this.loginCredentials).subscribe(allowed => {
if (allowed) {
setTimeout(() => {
this.loading.dismiss();
this.nav.setRoot(HelloIonicPage)
});
} else {
this.showError("Access Denied");
}
},
error => {
this.showError(error);
});
}
auth is a service provider, that has login method.
//Function to get access token
public login(credentials) {
if (credentials.email === null || credentials.password === null) {
return Observable.throw("Please insert credentials");
} else {
return Observable.create(observer => {
var link = 'http://localhost/XXX/public/oauth/token';
var vars = {
password: "XXX",
username: "XXXXXX",
grant_type: 'password',
client_id: "XXXXX",
client_secret: 'XXXXXX',
scope: ''
}
this.http.post(link, vars)
.map(res => res.json())
.subscribe(
data => { let user = this.getUserFromAccessToken(data);
console.log(user);
observer.next(user);
},
err => { observer.error(err.json()); }
() => {
console.log('Completed..');
}
);
});
}
}
//Function to get user from the accessToken
private getUserFromAccessToken(oAuthData) {
let headers = new Headers({ 'Content-Type': 'application/json','Accept': 'application/json','Authorization': 'Bearer ' + oAuthData.access_token });
let options = new RequestOptions({ headers: headers });
let link = 'http://localhost/XXXX/public/api/v1/user';
return this.http.get(link, options)
.map(res => res.json())
.subscribe(
data => this.currentUser = data.user,
err => this.error = err
);
}
currentUser and error are defined as properties of the AuthService class.
How should I ensure that an error is thrown either in case the access token is not returned or user is not returned from the access token.