Resolve "Follow-up from "Productivity Analytics: Add error handling for reporting on groups which have no plan""

What does this MR do?

This MR cleans up some code for Productivity Analytics.

Initially, we stored a Boolean on the state's hasError property since we didn't care about the error code. In !15291 (merged), we changed this behavior and are now setting the error code on the state. The state's property name hasError wasn't changed and might be misleading since it is now set to a number instead of a boolean. This MR changes the property name to errorCode. In addition, it updates its value to null when there is no error.

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

Closes #13836 (closed)