Zebra returns a confusing error for block proposals when it isn't fully synced yet

[teor2345]

Motivation

Sometimes Zebra knows that it isn't fully synced, but it accepts block proposals anyway.

This is a temporary issue that gets resolved when Zebra syncs to the tip. And it's just in proposal mode. So this seems like a low priority.

The proposals fail with an error like:
(The error format was changed in PRs #5993 and #6044.)

invalid proposal: Block {
    source: ValidateProposal(
        "proposal is not based on the current best chain tip: previous block hash must be the best chain tip",
    ),
}

But it would be more helpful to return the same error as the template mode:

Zebra has not synced to the chain tip, estimated distance: 67, local tip: Height(1952293). Hint: check your network connection, clock, and time zone settings.

Specifications

We might want to follow whatever zcashd does here.
This might impact compatibility with some mining pools.

Testing

Manually run zcash-rpc-block-template-to-proposal. To reproduce this error, get the template from a synced zcashd instance, and send it to a Zebra instance you just started.

Related Work

These tickets change how errors are displayed:

• Use the same format as zcashd for getblocktemplate proposal mode errors  #5981

[mpguerra]

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