Replace BaseForm.errors property with get_errors method

[Artem Rizhov]

This is not a real bug, but this makes some problems. I think property getter should not do so much changes into the internal state of the object. This getter calls the full_clean method, so this is actually not a getter, but a real worker.

This also produces very annoying side effects on debugging which really look and work like a bug. Please see this stackoverflow question for example
​http://stackoverflow.com/questions/12142819/debugging-form-validation-in-pycharm
(sorry for the link to third-party resource, please let me know if you think this text should be copied here)

I propose to replace errors property with get_errors method so that

• anybody can see it does some work to retrieve the errors
• debuggers do not try to evaluate it
• developers do not spend hours for catching non-existent bugs in their apps :)