[5.6] Add assertion to verify there are no errors

[mnabialek]

This allows to verify if there are errors without looking at Framework details. Now inside assertSessionHasErrors(), there is call:

$this->assertSessionHas('errors');

but if you want to make sure there are no errors, you need to really know that there shouldn't be errors key in session. After adding this method you don't need to care how it's implemented any more.