Set InitMessage for txindex migration

Summary:
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

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

error code: -28
error message:
Upgrading txindex database

Reviewers: jasonbcox, Fabien, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3115