Display all VSA errors on UI upon value stream form validation

What does this MR do and why?

Intercepts minimum character length validation for value stream name on the frontend, and displays any additional value stream name or custom stage errors from backend on UI. Also refactors/reorganizes value_stream_form_content_spec.js a bit.

Screenshots or screen recordings

	Before	After
Min value stream name char length validation
Value stream name backend errors
Custom stage name backend errors

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Please make sure that you're on the GitLab Ultimate plan.
2. Visit VSA for any group or seed data if necessary.
3. Create/edit value stream and test that correct field errors display in following scenarios:
   1. Value stream name less than three characters
   2. Duplicate value stream name within same group (backend error)
4. Edit value stream and test that correct field errors display in following scenarios:
   1. Duplicate custom stage name within same value stream (backend error)

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.

Related to #407829 (closed)