Handle invalid import URLs

What does this MR do and why?

Update service to handle invalid URLs

The service was refactored in !190545 (merged) which caused the Import::UrlController#validate to raise 500 errors for invalid URLs.

This change handles the errors and returns a proper message to the users.

References

#536989 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open /projects/new#import_project page
2. Click on the Repository by URL button
3. Enter an invalid URL - verify that an error is displayed in the UI

MR acceptance checklist

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