[24.2] Provide guidance in case of deadlock during db migration

[jdavcs]

Ref: #19512
This will catch all PostgreSQL deadlock errors during migrations and will display a helpful message. Also added a troubleshooting note to Galaxy's db migration documentation.

Also fixes downgrading to a unique index on role.name.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.