images: catch unicode errors

When the headers of the TarFile have non-utf8 encoded values (such is
the case for some xattrs for file capabilities) (see
http://ur1.ca/gzn2m), the repository fails to get pushed. Instead, just
do not generate the tarsum for images with this failure.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Merge pull request #306 from vbatts/vbatts-catch_unicode_error

images: catch unicode errors