Reject a `location.reference` that doesn't select an element

[cjbarth]

Split out of #583. In 6.3.3, #583 fixes the errors for location.reference values that already throw. This issue is the other half of its suggested fix: refusing a location.reference that doesn't select an element.

Today, before and after accept a text node, a comment or a processing instruction and put the Signature next to it. Probed on master at c13e30b, signing <root id="x"><a>t</a>text<!--c--><?pi d?></root>:

location.reference selects	append / prepend	before / after
a text node	xmldom Hierarchy request error	signs
a comment	xmldom Hierarchy request error	signs
a processing instruction	xmldom Hierarchy request error	signs

Proposal

Throw a named error from computeSignature() when location.reference selects anything other than an element, whatever the action.

Why 7.0

before and after with these nodes sign today.