Improve error handling of runner instructions

What does this MR do?

Improve error handling of runner instructions modal

This change improves the error handling when instructions cannot be loaded or are empty.

It also fixes a case for sub-groups by using full_path.

Screenshots (strongly suggested)

Before:

• Console error appears as user has not acces, but this is not handled correctly
• Alert is not shown

After:

• Console error is gone
• Alert is shown

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
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• Style guides
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• 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

Related to #326018