onSignInMethodExists = function() {
    var errorMessage =
        firebaseui.auth.widget.handler.common.getErrorMessage(emailExistsError);
    firebaseui.auth.ui.element.setValid(component.getEmailElement(), false);
    firebaseui.auth.ui.element.show(
        component.getEmailErrorElement(), errorMessage);
    component.getEmailElement().focus();
  }
Process the email exists error.
@param {firebaseui.auth.AuthUI} app The current Firebase UI instance whose
    configuration is used.
@param {firebaseui.auth.ui.page.PasswordSignUp} component The UI component.
@param {string} email The current email.
@param {*} emailExistsError The email exists error.
@return {!firebase.Promise} The promise that resolves when email exists error
    is handled.
@private