Give reason while file open failed.

src/ports/SkOSFile_stdio.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkOSFile.h"
 #include "SkTypes.h"
 
@@ skipping 14 matching lines @@
         *p++ = 'r';
     }
     if (flags & kWrite_SkFILE_Flag) {
         *p++ = 'w';
     }
     *p++ = 'b';
     *p = 0;
 
     //TODO: on Windows fopen is just ASCII or the current code page,
     //convert to utf16 and use _wfopen
-    return (SkFILE*)::fopen(path, perm);
+    SkFILE* file = (SkFILE*)::fopen(path, perm);
+    if (nullptr == file && (flags & kWrite_SkFILE_Flag)) {
+        SkDEBUGF(("sk_fopen: fopen(\"%s\", \"%s\") returned NULL (errno:%d): %s\n",
+                  path, perm, errno, strerror(errno)));
+    }
+    return file;
 }
 
 char* sk_fgets(char* str, int size, SkFILE* f) {
     return ::fgets(str, size, (FILE *)f);
 }
 
 int sk_feof(SkFILE *f) {
     // no :: namespace qualifier because it breaks android
     return feof((FILE *)f);
 }
@@ skipping 100 matching lines @@
 #else
     retval = mkdir(path, 0777);
 #endif
     if (0 == retval) {
         return true;
     } else {
         fprintf(stderr, "sk_mkdir: error %d creating dir '%s'\n", errno, path);
         return false;
     }
 }