Add extra output to diagnose file creation failure

printer->Initialize(output_path) sometimes fails and we don't know why
so step one is to print some extra information when it fails.

Bug: 834736
Change-Id: I65bda362d38c0899542da9234e0b47ef57d1aa56
Reviewed-on: https://chromium-review.googlesource.com/1121252
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573038}

base/test/test_suite.cc

@@ -243,7 +243,9 @@ void TestSuite::AddTestLauncherResultPrinter() {
   }
 
   printer_ = new XmlUnitTestResultPrinter;
-  CHECK(printer_->Initialize(output_path));
+  CHECK(printer_->Initialize(output_path))
+      << "Output path is " << output_path.AsUTF8Unsafe()
+      << " and PathExists(output_path) is " << PathExists(output_path);
   testing::TestEventListeners& listeners =
       testing::UnitTest::GetInstance()->listeners();
   listeners.Append(printer_);