Improve block validation for signed messages

[JJ12880]

Signed message validation should check that the block included in the message actually exists.
A failure/error dialog should show if the block does not exist.

A warning message should show, if the signed message is old. Perhaps just a warning icon by the date in the valid message dialog.

https://github.com/RadiumCore/zeroclient_gui/blob/master/ClientApp/components/MainPages/SignedMessages/VerifiedMessagePopup.tsx