Rescue PreReceiveError in PullRequest GitHub importer

What does this MR do and why?

This MR updates PullRequestImporter#create_source_branch_if_not_exists to not fail when Gitlab::Git::PreReceiveError exception is raised. This method is used during Project Import from GitHub and if something goes wrong the entire project import is failed. Rescue this exception in order to allow user to get their project imported.

It fixes Gitlab::GithubImport::ImportPullRequestWorker: Rescue PreReceiveError in create_source_branch_if_not_exists.

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