Fix visiting and saving writable content provider files

* java/org/gnu/emacs/EmacsService.java (checkContentUri):
Improve debug output.
* lisp/files.el (basic-save-buffer): Check whether or not file
itself exists before checking for the existence of the directory
containing it.
* src/android.c (android_open): Don't forget to set errno after
open_content_uri fails.

java/org/gnu/emacs/EmacsService.java

@@ -752,6 +752,8 @@ public final class EmacsService extends Service
     if (writable)
       mode += "w";
 
+    Log.d (TAG, "checkContentUri: checking against mode " + mode);
+
     try
       {
 	fd = resolver.openFileDescriptor (Uri.parse (name), mode);