Improve the legacy chain panic message

[teor2345]

Motivation

We implemented a legacy chain check in #2366.

But it would help users if we told them the invalid network upgrade, the expected network upgrade, and the failing height.

We should also see what happens if someone installs an old version, then tries to open a new state. (I think the old version will panic when it parses an unrecognised branch id in a transaction.)

Priority

This situation should be reasonably rare, so usability improvements are a low priority.

Specifications

If a user does not upgrade their node to a compatible software version before ACTIVATION_HEIGHT is reached, and the node continues running (which could normally only occur if the End-of-Service halt were bypassed), then the node will follow any pre-upgrade consensus branch that persists. In this case it may download blocks that are incompatible with the post-upgrade consensus branch.
If the user subsequently upgrades their node to a compatible software version, the node will consider these blocks to be invalid, and if there are a significant number of invalid blocks it SHOULD shut down and alert the user of the issue.

https://zips.z.cash/zip-0200#post-activation-upgrading

Designs

The current design is documented in the legacy_chain_check function.

[teor2345]

This change isn't needed.