Update assert message

generators/chipyard/src/main/scala/stage/ChipyardAnnotations.scala

@@ -13,7 +13,7 @@ private[stage] object UnderscoreDelimitedConfigsAnnotation extends HasShellOptio
       longOption = "legacy-configs",
       toAnnotationSeq = a => {
         val split = a.split(':')
-        assert(split.length == 2)
+        assert(split.length == 2, s"'${a}' split by ':' doesn't yield two things")
         val packageName = split.head
         val configs     = split.last.split("_")
         Seq(new ConfigsAnnotation(configs map { config => if (config contains ".") s"${config}" else s"${packageName}.${config}" } ))