This makes it so that when RPC calls are tried during the initial migration
(which takes ~ an hour on mainnet) they error with "-28 Upgrading txindex database"
instead of the misleading "-28 Verifying blocks...".

(why it's needed: only InitMessage is connected to SetRPCWarmupStatus)

Depends on D2924

build; try it out
On testnet I confirm that during migration, running bitcoin-cli getinfo gives:

error code: -28
error message:
Upgrading txindex database