core: Sanitize error text validating refs (e.g. against HTML)

core: Sanitize error text validating refs (e.g. against HTML)

See: https://github.com/projectatomic/rpm-ostree/issues/885

If we get a successful Apache directory listing HTML when fetching what we
intend to be a ref, we'd dump the HTML into the error.

I did some scanning of the pull code, and this was the only case
I saw offhand where we were dumping text out into an error.  Which
makes sense, since most of our formats are binary, the exeptions I
think are just `repo/config` and `repo/refs/`.

Closes: #1015
Approved by: mbarnes