GHA: Fix dumb error check

Previously the reply JSON was examined for the literal presence of the
string 'error'.  This was intended to catch server or query errors and
the like.  However it's not a sound design as valid/legitimate contents
could potentially contain the string.  Fix this by using the `-e` option
to `jq`, with a filter that should always result in a non-empty/null
match.  If this fails or returns null for some reason, then it's safe to
throw a real error code & message.

Signed-off-by: Chris Evich <[email protected]>