adds error checking when fetching dynamic vars
adds error checking when fetching dynamic vars

Previously this method would fail if a host were down,
but return a string anyway. Updated version checks $?
and quits if false.

I don't like the stderr handling in this function.

mark-in/securedrop-prov-upstream,mark-in/securedrop-prov-upstream,mark-in/securedrop-prov-upstream,mark-in/securedrop-prov-upstream