Surface `another change is still in progress` messages in DB Console

**Is your feature request related to a problem? Please describe.**
Currently in DB Console, there is only the ability to view a job that is running with no indication that it is waiting for something else indefinitely.

**Describe the solution you'd like**
Surface the `another change is still in progress` messages in DB Console for schema changes so the user knows why a job is hanging.

Jira issue: CRDB-24672

Epic CRDB-32494

Start with the DB Console entry point for viewing running schema-change jobs and trace where their status is presented. Identify the source of the `another change is still in progress` message, then verify that a waiting job exposes that message and explains why it is not progressing.

Written by the indexing model from the issue text.