Added seeFormErrorMessage function

[TavoNiievez]

     $I->seeFormErrorMessage('username');
     $I->seeFormErrorMessage('username', 'Username is empty');

Also present in the Laravel module... But a bit more complex 😃.

• Fails if there are no forms when asserting.
• Fails if an invalid field is passed as an argument.
• It performs its function by verifying that the field is valid and has errors.
• If the expected error message is specified it verifies that it matches the current one. (technically with: ->assertStringContainsString()).

[DavertMik]

Good. Once it is merged we should have corresponding tests in symfony-demo