Specify failing dependencies in the callout message

What does this MR do?

To help users pinpoint the cause of the missing dependencies failure, we want to improve the callout failure message shown on the job's details page.

We now include the list of jobs on which the dependencies have failed. We also include a link to the documentation about when dependencies fail.

To avoid any unexpected side-effects, I only modified the BuildDetailsEntity serializer because we only want this message on the job's details page.

Related discussion: #6144 (comment 222127613)

Solves #6144 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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