Add context to error message.

Would have been very useful to avoid http://b/227682097

Bug: 227682097

Test: m apexer apexer_test deapexer debugfs_static; atest --host apexer_test
Change-Id: I24234d053a06868493831d40f94f7e46956391a1

tools/aapt2/util/Files.cpp

@@ -349,7 +349,7 @@ Maybe<std::vector<std::string>> FindFiles(const android::StringPiece& path, IDia
   const std::string root_dir = path.to_string();
   std::unique_ptr<DIR, decltype(closedir)*> d(opendir(root_dir.data()), closedir);
   if (!d) {
-    diag->Error(DiagMessage() << SystemErrorCodeToString(errno));
+    diag->Error(DiagMessage() << SystemErrorCodeToString(errno) << ": " << root_dir);
     return {};
   }