Returns helpful msg in case requirements file does not exist,
    or cannot be parsed.

    :params req: Requirements file path

def deduce_helpful_msg(req):
    # type: (str) -> str
    """Returns helpful msg in case requirements file does not exist,
    or cannot be parsed.

    :params req: Requirements file path
    """
    msg = ""
    if os.path.exists(req):
        msg = " It does exist."
        # Try to parse and check if it is a requirements file.
        try:
            with open(req, 'r') as fp:
                # parse first line only
                next(parse_requirements(fp.read()))
                msg += " The argument you provided " + \
                    "(%s) appears to be a" % (req) + \
                    " requirements file. If that is the" + \
                    " case, use the '-r' flag to install" + \
                    " the packages specified within it."
        except RequirementParseError:
            logger.debug("Cannot parse '%s' as requirements \
            file" % (req), exc_info=True)
    else:
        msg += " File '%s' does not exist." % (req)
    return msg