Little improvement of errors show on volume Extend form

[Zhenguo Niu]

Now when extending a volume to a new size which is less then the current
one, we will pop up an error warning message on the top of the form instead
of beside the new size field. now change to use field specified error class
for more accurate.

And as Kirill Zaitsev suggested, it
would be even better if we could incorporate https://docs.djangoproject.com/en/1.8/ref/forms/api/#django.forms.Form.add_error, which would not make us fiddle with form internals. but it was only added in django 1.7, and we support up to 1.5 but moving forward with django support seems like a nice idea.

[Zhenguo Niu (niu-zglinux)]

https://review.openstack.org/#/c/200158/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/200158
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=5d45cef785be30265ea65685bc798c7ef3f772e3
Submitter: Jenkins
Branch: master

commit 5d45cef785be30265ea65685bc798c7ef3f772e3
Author: Zhenguo Niu <email address hidden>
Date: Thu Jul 9 23:42:34 2015 +0800

    Use form field errors on volume Extend form

    Now when extending a volume to a new size which is less then the current
    one, we will pop up an error warning message on the top of the form instead
    of beside the new size field. now change to use field specified error class
    for more accurate.

    Closes-Bug: #1480001
    Change-Id: I2ee1a562bf548e78d657a3c5bd3307eacf6a3ab1