Project import Github API rate limit is exceeded

What does this MR do?

related to: #26466 (closed)

The problem

1. On gitlab, Go to https://gitlab.com/projects/new
2. Click on "Import Project" tab
3. Click on Github (I had it previously authorized, so no authentication was required)
4. Got a 500 error - if the user doing the import exceeded the Github API rate limit (https://developer.github.com/v3/#rate-limiting)

The solution

When importing a project from Github, if the Github API rate limit is exceeded, show an error with the "reset_time" to the user, instead of a 500 error.

Follow up

This MR goal is to do the minimal change to fix the described problem. Another issue was created to discuss the improvement on this flow.

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