Sync - recognise existing PRs and treat as success

[ewels]

See #709 (comment)

Need to catch the following error:

ERROR     Sync failed for nf-core/ampliseq:                          sync.py:385
GitHub API returned code 422:
{
    "message": "Validation Failed",
    "errors": [
        {
            "resource": "PullRequest",
            "code": "custom",
            "message": "A pull request already exists for nf-core:TEMPLATE."
        }
    ],
    "documentation_url": "https://developer.github.com/v3/pulls/#create-a-pull-request"
}

and treat it as a success - in this scenario the PR to update the template already exists, and our new commit is appended to it.

[ewels]

Ooh, even better - bioconda bot recognises this and then renames the PR title:

For example: bioconda/bioconda-recipes#23469

[ewels]

Closed in #716