Show errors in errorless test

Author: som-snytt

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

@@ -947,16 +947,22 @@ trait ParallelTesting extends RunnerOrchestration:
 
       Option {
         if actualErrors == 0 then s"\nNo errors found when compiling neg test $testSource"
-        else if expectedErrors == 0 then s"\nNo errors expected/defined in $testSource -- use // error or // nopos-error"
+        else if expectedErrors == 0 then
+          s"""|
+              |No errors expected/defined in $testSource -- use // error or // nopos-error"
+              |$showErrors
+              |""".stripMargin.trim.linesIterator.mkString("\n", "\n", "")
         else if expectedErrors != actualErrors then
           s"""|Wrong number of errors encountered when compiling $testSource
               |expected: $expectedErrors, actual: $actualErrors
               |${expected.mkString("Unfulfilled expectations:\n", "\n", "")}
               |${unexpected.mkString("Unexpected errors:\n", "\n", "")}
               |$showErrors
               |""".stripMargin.trim.linesIterator.mkString("\n", "\n", "")
-        else if hasMissingAnnotations then s"\nErrors found on incorrect row numbers when compiling $testSource\n$showErrors"
-        else if !errorMap.isEmpty then s"\nExpected error(s) have {<error position>=<unreported error>}: $errorMap"
+        else if hasMissingAnnotations then
+          s"\nErrors found on incorrect row numbers when compiling $testSource\n$showErrors"
+        else if !errorMap.isEmpty then
+          s"\nExpected error(s) have {<error position>=<unreported error>}: $errorMap"
         else null
       }
     end maybeFailureMessage