Resolve "More descriptive error messages in Migration Helpers"

What does this MR do?

Improve the error messages for GitLab custom migration helpers to point out certain failures due to schema inconsistencies. For example: A migration attempts to rename a column, but for some reason the old column doesn't exist. The current behavior displays an error message of undefined method type' for nil:NilClass`. This MR changes that to display a more useful error message that highlights the underlying schema problem and a possible resolution.

Linked issue #207126 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• 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