be more tolerant of tag read failures

I am running into this issue on a file with a non-conforming MakerNote field that has a pointer to outside the EXIF block.  For what it's worth, both libexif and exiv2 are more tolerant of this case by just printing an error.  It seems that this situation can be very common as discussed here: http://www.exiv2.org/makernote.html