Print missing ref diagnostics on stderr

Was stdout, but this gets mixed up with the exception printing on stderr.

Author: odersky

src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -419,8 +419,8 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
               // println(owner.info.decls.toList.map(_.debugString).mkString("\n  ")) // !!! DEBUG
               //              }
               // (5) Create a stub symbol to defer hard failure a little longer.
-              println(i"***** missing reference, looking for $name in $owner")
-              println(i"decls = ${owner.info.decls}")
+              System.err.println(i"***** missing reference, looking for $name in $owner")
+              System.err.println(i"decls = ${owner.info.decls}")
               new Exception().printStackTrace()
               ctx.newStubSymbol(owner, name, source)
             }