Improve error messages when CI blob is not loaded

What does this MR do?

This MR improves error messages when blob is not loaded.

This change improves usability for users when the yml file is not defined, it does this by changing the error message and hides controls that don't work in the editor.

Note: This change requires the flags ci_pipeline_editor_page to be enabled, so no changelog is included.

Screenshots (strongly suggested)

Visit CI/CD -> Editor.

Instead of showing a confusing error message and showing a form that cannot be used, this change hides the elements that can be used.

before	after

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
  • See !51461 (merged)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team