[web] Move modal_form_error within tab_body

Summary:
We want the error message to appear "within" the tab_body so it isn't pushed to the outer edge of the Modal. This required "copy/pasting" the error message div three times... couldn't think of a way around this because only Tabs.Item components can be nested within Tabs.Container.

Note: As an aside, I think displaying the error message within the "Save" button could be cleaner and keep the layout simpler. We do something like this on landing with the SubscriptionForm component. Not saying we should prioritize this right now, but just thought it might look cleaner. Might looks something like:

Depends on D3933

Test Plan:
Here's what it looks like:

Before:

After:

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3934