Handle network errors / make it possible to alter request on error

[valoricDe]

I use it like this:

var token = localStorage.getItem('jwtToken');

const g = require('../../misc/settings').graphql;
Relay.injectNetworkLayer(
	new RelayNetworkLayer('http://'+g.host+':'+g.port+'/graphql', {
		headers: (token ? {
			Authorization: 'Bearer ' + token
		} : {}),
		onError: (error, init) => {
			console.log(error);
			if(error.status == 400 || error.status == 500) {
				localStorage.removeItem('jwtToken');
				delete init.headers['Authorization'];
				console.log('removed token');
				return true; // directly retry request
			}
			return false;
		}
	})
);

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