REPL: do not report warnings when there are errors

#KT-18349 Fixed

compiler/testData/repl/controlFlow/incompleteWhen.repl

@@ -2,9 +2,6 @@
 error: 'when' expression must be exhaustive, add necessary 'else' branch
 val a = when(null) { }
         ^
-warning: the expression is unused
-val a = when(null) { }
-             ^
 >>> "" + when (true) { true -> 42 }
 error: 'when' expression must be exhaustive, add necessary 'false' branch or 'else' branch instead
 "" + when (true) { true -> 42 }

compiler/testData/repl/noWarningsWithErrors.repl

@@ -0,0 +1,11 @@
+>>> (listOf(1) as List<String>).unresolved()
+error: unresolved reference: unresolved
+(listOf(1) as List<String>).unresolved()
+                            ^
+>>> listOf<String>(1!!)
+error: the integer literal does not conform to the expected type String?
+listOf<String>(1!!)
+               ^
+error: type mismatch: inferred type is Int but String was expected
+listOf<String>(1!!)
+               ^