Fix import_data being saved as a result of an invalid import_url

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/19479

Seem that in all cases the `import_url` was an empty string. The `import_url` is now validated correctly and no longer creates the `import_data`even if it fails validation. <= Should cover cases 1, 3, and 4 of https://gitlab.com/gitlab-org/gitlab-ce/issues/19479#note_13005276

Also, it now rescues from all exceptions when importing from a URL, so we make sure the `import_status` is failed after that. Covers case 2 of https://gitlab.com/gitlab-org/gitlab-ce/issues/19479#note_13005276

See merge request !5206