Complete 'pdf_extractor.lambda_handler' tests

[will-afs]

    except ConnectionRefusedError:
        status = 502
        message = 'Internal service CooldownManager did not allow to request ArXiv.org services'
        body = {}
    except UnknownDocumentTypeError:
        status = 400
        message = 'Please provide an URI pointing to a PDF'
        body = {}