Don't insert an <outdent> where a statement is logically continued

Thsi fixes a case observed in SJSCodeGen, which was basically like this:

```scala
      if (x) ...
      else /*
      bla
      bla
    } else */ {
```
The problem is that the commented out `}` in front of the uncommented `{` caused
an <outdent> to be inserted.

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -588,7 +588,10 @@ object Scanners {
         if (nextWidth < lastWidth
             || nextWidth == lastWidth && (indentPrefix == MATCH || indentPrefix == CATCH) && token != CASE)
           currentRegion match {
-            case r: Indented if !r.isOutermost && !isLeadingInfixOperator() =>
+            case r: Indented
+            if !r.isOutermost &&
+               !isLeadingInfixOperator() &&
+               !statCtdTokens.contains(lastToken) =>
               currentRegion = r.enclosing
               insert(OUTDENT, offset)
               handleEndMarkers(nextWidth)

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -578,7 +578,7 @@ object messages {
   case class IllegalStartSimpleExpr(illegalToken: String)(implicit ctx: Context)
   extends Message(IllegalStartSimpleExprID) {
     val kind: String = "Syntax"
-    val msg: String = "expression expected"
+    val msg: String = em"expression expected but ${Red(illegalToken)} found"
     val explanation: String = {
       em"""|An expression cannot start with ${Red(illegalToken)}."""
     }