Currently, the Github Importer might end up stuck even after failing to
import the git repository. This might also happen when importing other
types of objects from Github.

To provide better feedback to the end user this kind of exception will
now mark the import (import_state) as failed. Also, to provide
information to developers and testers the exceptions errors will be
logged (importer.log) and saved in the import_failures table.

== What was done

* Created Gitlab::Import::ImportFailureService: This way we now have a
  standard way to log exceptions/errors that happens while importing a
  project;
  * All the exceptions/errors are logged to the importer.log with a
    standard format;
  * All the exceptions/errors are tracked in Gitlab::ErrorTracking;
  * All the exceptions/errors are recorded in the import_failures table;
  * When required, the exception/error might mark the whole import as
    failed;
  * This was created in a more generic way because it can later be used
    in the other importers (bitbucket, gitlab, etc);

* Updated Github Importer workers/importers to leverage of the
  Gitlab::Import::ImportFailureService;

* Updated Gitlab::GithubImport::Queue, to use
  Gitlab::Import::ImportFailureService when jobs exhausted retries;

* Updated Gitlab::Import::StuckImportJob to use
  Gitlab::Import::ImportFailureService;
  * This is also used in the Jira importer;

Related to: gitlab-org/gitlab#335660

Changelog: fixed
MR: gitlab-org/gitlab!67454