[DEV-8492] Fail lv:param-class-to-yields rather than awaiting propagation

This problem manifested when the name of the attempted classification is the
same name as another object.  For example, if we have `t:match-class
name="foo"`, and `foo` is a param instead of a class, then `@yields` will
fail, and it'd fall back to matching on the param.

This is absolutely not what we want.

The error message in this context is ugly, but it does work.

Example:

  !!! Unknown match @on (/lv:package/lv:classify/match): `error: unable to
  determine @yields for class `scheduled_ai' (has the class been imported?)'
  is unknown for classification --vis-scheduled-ai-type

RELEASES.md

@@ -16,6 +16,11 @@ commits that introduce the changes.  To make a new release, run
 
 NEXT
 ====
+Compiler
+--------
+- `lv:param-class-to-yields` will now trigger a failure rather than relying
+  on propagating bad values, which may not result in failure if the symbol
+  is represented by another type (non-class) of object.
 
 Miscellaneous
 -------------

src/current/include/preproc/template.xsl

@@ -1264,16 +1264,11 @@
 
   <choose>
     <when test="not( $yields ) or $yields=''">
-      <message>
+      <preproc:error>
         <text>error: unable to determine @yields for class `</text>
         <value-of select="$as" />
         <text>' (has the class been imported?)</text>
-      </message>
-
-      <!-- just retain the name; it'll be used for an error message,
-           since it won't be foudn -->
-      <!-- TODO: this is dangerous; find a way to propagate the error -->
-      <value-of select="$as" />
+      </preproc:error>
     </when>
 
     <otherwise>