Fix flaky test detector failing on missing file (open-telemetry#13172)

Author: laurit

test-report/src/main/java/io/opentelemetry/instrumentation/testreport/FlakyTestReporter.java

@@ -178,6 +178,13 @@ public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) {
 
             return CONTINUE;
           }
+
+          @Override
+          public FileVisitResult visitFileFailed(Path file, IOException exception) {
+            System.err.println("Failed to visit " + file.toString());
+            exception.printStackTrace();
+            return CONTINUE;
+          }
         });
   }