Makes error message clearer

What does this MR do and why?

Error message was introduced in !131141 (merged) but does not clarify the user action required.

Expands the error message to users to make it more clear what they need to do.

How to set up and validate locally

1. Import a project that has pipelines.
2. Try to start a pipeline while project is still importing.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.