Improve message if submit fails due to non-fulfilled submit requirements
Improve message if submit fails due to non-fulfilled submit requirements

If a submit request fails due to non-fulfilled submit requirements, the
response only contains the fallback text of the submit requirement.
Without letting the user know that this message is about a non-fulfilled
submit requirement, the message may be unclear to the caller.

The fallback text is usually kept as short as possible, since the
frontend is displaying it in the UI. Hence it cannot be made more
verbose without affecting the UI.

Signed-off-by: Edwin Kempin <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>
Change-Id: Iecd9304aa98efa6f404da9a50d2e002c6aefc6f3

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