WEBP exif data

[Hesarn]

Hi

it's not clear for me why should an exception should be raised if exif data is not present, it can be initialized like jpeg loader or at least a dedicated exception can be raised instead. The current behavior is making it hard to handle if there is no exif.

def get_exif(data):
    if data[0:4] != b"RIFF" or data[8:12] != b"WEBP":
        raise ValueError("Not WebP")

    if data[12:16] != b"VP8X":
        raise ValueError("doesnot have exif")

i think this behavior in jpeg loader, is more natural:

def load(input_data, key_is_name=False):
    """
    py:function:: piexif.load(filename)

    Return exif data as dict. Keys(IFD name), be contained, are "0th", "Exif", "GPS", "Interop", "1st", and "thumbnail". Without "thumbnail", the value is dict(tag name/tag value). "thumbnail" value is JPEG as bytes.

    :param str filename: JPEG or TIFF
    :return: Exif data({"0th":dict, "Exif":dict, "GPS":dict, "Interop":dict, "1st":dict, "thumbnail":bytes})
    :rtype: dict
    """
    exif_dict = {"0th":{},
                 "Exif":{},
                 "GPS":{},
                 "Interop":{},
                 "1st":{},
                 "thumbnail":None}
    exifReader = _ExifReader(input_data)
    if exifReader.tiftag is None:
        return exif_dict