Simple Payments: display better error when Paypal email address is unconfirmed

This was reported by a site owner in #17965472-hc when attempting to fill the email form in the Simple Payments Block, as soon as the email address is added the block converts to a Paragraph block, and all data entered in the Simple Payments Block is lost.

#### Steps to reproduce the issue

1. Create a new page and set up a simple payments block
2. Add a valid email address (attached to a PayPal account) in the email field
3. Immediately upon entering email the block settings are lost

#### What I expected
To be able to add the email address and continue to configure the block

#### What happened instead
All block contents entered prior to the email address are lost. Block is no longer a Simple Payment but a paragraph

Video if the issue in action: https://d.pr/v/2858kF

Issue has been replicated on an Atomic site running Jetpack, as well as on wpcom (Calypso) when using the Gutenberg editor on a wpcom simple site. There does not seem to be any way to successfully save and publish the Payments block
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Start by reproducing the Simple Payments Block failure in the Gutenberg editor with an unconfirmed PayPal email, using the steps and video in this issue. Compare behavior on Atomic and wpcom sites as described. Done means the block no longer converts to a Paragraph or loses entered data, and the user receives an appropriate error when the email is unconfirmed.

Written by the indexing model from the issue text.