When a test fails report number of _failures_ not _errors_

Right now we report the number of "errors", that is compiler errors including
the expected ones, but we should instead use the test "failures", that is the
unexpected behaviors.

Also make errorCount private.

Author: Blaisorblade

compiler/test/dotty/tools/vulpix/ParallelTesting.scala

@@ -227,7 +227,8 @@ trait ParallelTesting extends RunnerOrchestration { self =>
     val sourceCount = filteredSources.length
 
     private[this] var _errorCount =  0
-    def errorCount: Int = _errorCount
+    /** Number of (expected or unexpected) compiler errors. Unlikely to be what you want, use `failureCount` instead. */
+    private[this] def errorCount: Int = _errorCount
 
     private[this] var _testSourcesCompleted = 0
     private def testSourcesCompleted: Int = _testSourcesCompleted
@@ -1013,8 +1014,8 @@ trait ParallelTesting extends RunnerOrchestration { self =>
     /** Extract `Failure` set and render from `Test` */
     private[this] def reasonsForFailure(test: Test): String = {
       val errors =
-        if (test.errorCount == 0) ""
-        else s"\n  - encountered ${test.errorCount} error(s)"
+        if (test.failureCount == 0) ""
+        else s"\n  - encountered ${test.failureCount} error(s)"
 
       errors + test.failureReasons.collect {
         case test.TimeoutFailure(title) =>