Resolve "NoMethodError: undefined method `error' for #<Import::ValidateRemoteGitEndpointService:0x00007fe2e6106320>

What does this MR do and why?

This MR fixes wrong error call which originally slipped through review in !70165 (merged) and replaces it with proper invocation of ServiceResponse.error

• Test included

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Open /projects/new#import_project
2. Select "Repo by URL"
3. Provide username / credentials and ensure that error message appears / disappears correctly

For testing basic auth (repo with credentials):

1. Generate new token with read_repository permissions on gitlab.com
2. Use any private repository URL to test access (note: username is ignored in gitlab when using personal access token as password, this is known

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #342323 (closed)