[TERMREF::myterm] should issue an error if myterm is not defined in glossary

Currently, `[TERMREF::myterm]` simply evaluates to a link to `glossary.html#myterm`, even if there is no such term and anchor in the glossary.

Instead, `sedrila author` should know which glossary entries exist and complain about a missing one, clearly mentioning where it found the broken `[TERMREF]`.