Fix #2791: add reasons for failure to vulpix

[felixmulder]

No description provided.

[felixmulder]

Fails on something that seems unrelated:

[warn] -- [E028] Pattern Match Exhaustivity Warning: /drone/src/github.com/lampepfl/dotty/doc-tool/src/dotty/tools/dottydoc/model/references.scala:40:32 
[warn] 40 |    def showReference: String = ref match {
[warn]    |                                ^^^
[warn]    |match may not be exhaustive.
[warn]    |
[warn]    |It would fail on: EmptyReference, references.AndTypeReference(_, _), references.OrTypeReference(_, _)
[warn] one warning found
java.lang.AssertionError: assertion failed: ImplicitMethodType(List(summaryReport), List(TypeRef(ThisType(TypeRef(NoPrefix,vulpix)),SummaryReporting)), TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,vulpix)),VulpixTests)),CompilationTest)) TypeRef(ThisType(TypeRef(NoPrefix,scala)),Unit)
	at scala.Predef$.assert(Predef.scala:170)
	at dotty.tools.dotc.core.Types$OrType.<init>(Types.scala:2388)
	at dotty.tools.dotc.core.Types$CachedOrType.<init>(Types.scala:2416)
	at dotty.tools.dotc.core.Types$OrType$.apply(Types.scala:2421)
	at dotty.tools.dotc.core.TypeComparer$$anonfun$orType$1.apply(TypeComparer.scala:1313)
	at dotty.tools.dotc.core.TypeComparer$$anonfun$orType$1.apply(TypeComparer.scala:1313)
	at dotty.tools.dotc.core.TypeComparer.dotty$tools$dotc$core$TypeComparer$$liftIfHK(TypeComparer.scala:1326)
	at dotty.tools.dotc.core.TypeComparer.orType(TypeComparer.scala:1313)
	at dotty.tools.dotc.core.TypeComparer$$anonfun$lub$2.apply(TypeComparer.scala:1197)
	at dotty.tools.dotc.core.TypeComparer$$anonfun$lub$2.apply(TypeComparer.scala:1182)
	at dotty.tools.dotc.reporting.Reporting$class.traceIndented(Reporter.scala:140)
	at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:57)
	at dotty.tools.dotc.core.TypeComparer.lub(TypeComparer.scala:1181)
	at dotty.tools.dotc.core.Types$Type$$anonfun$$bar$1.apply(Types.scala:849)
	at dotty.tools.dotc.core.Types$Type$$anonfun$$bar$1.apply(Types.scala:849)
	at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)
	at dotty.tools.dotc.core.Types$Type.$bar(Types.scala:848)
	at dotty.tools.dotc.typer.TypeAssigner$$anonfun$lubInSameUniverse$1$$anonfun$apply$7.apply(TypeAssigner.scala:555)
	at dotty.tools.dotc.typer.TypeAssigner$$anonfun$lubInSameUniverse$1$$anonfun$apply$7.apply(TypeAssigner.scala:555)
	at dotty.tools.dotc.typer.TypeAssigner$class.dotty$tools$dotc$typer$TypeAssigner$$inSameUniverse(TypeAssigner.scala:547)
	at dotty.tools.dotc.typer.TypeAssigner$$anonfun$lubInSameUniverse$1.apply(TypeAssigner.scala:555)
	at dotty.tools.dotc.typer.TypeAssigner$$anonfun$lubInSameUniverse$1.apply(TypeAssigner.scala:555)

[smarter]

LGTM, although it'd be nicer if we treated Java compiler errors like Scala compiler errors, but that would require using the Java API to compile Java code instead of running javac in a separate process