First-party auth doesn't return login errors

[eatyourgreens]

Login errors aren't returned as rejected promises, which leads to them being handled as resolved promises with undefined arguments. See zooniverse/Panoptes-Front-End#6236

Compare the error handler for auth.signIn here:

panoptes-javascript-client/lib/auth.js

Lines 227 to 230 in 49a56f4

	.catch(function(request) {
	console.error('Failed to sign in');
	apiClient.handleError(request);
	});

with the error handler for auth.register here:

panoptes-javascript-client/lib/auth.js

Lines 151 to 154 in 49a56f4

	.catch(function(request) {
	console.error('Failed to register');
	return apiClient.handleError(request);
	});

[eatyourgreens]

I think this bug was introduced by #164. Prior to that change, calling apiClient.handleError(request) would have thrown an error in the login form. Now that handleError returns a rejected promise, that return value has to be returned from the catch block in auth.signIn in order to be passed along the promise chain.