Improve copy-file diagnostics on MS-Windows.

src/fileio.c (Fcopy_file) [WINDOWSNT]: Improve diagnostics when CopyFile
 fails by looking at what GetLastError returns.

src/ChangeLog

@@ -3,6 +3,7 @@
 	* fileio.c (Fset_file_selinux_context, Fset_file_acl): Return t if
 	file's SELinux context or ACLs successfully set, nil otherwise.
 	(Bug#13298)
+	(Fcopy_file) [WINDOWSNT]: Improve diagnostics when CopyFile fails.
 
 	* w32proc.c (reader_thread): Avoid passing NULL handles to
 	SetEvent and WaitForSingleObject.

src/fileio.c

@@ -2029,7 +2029,15 @@ entries (depending on how Emacs was built).  */)
   if (!CopyFile (SDATA (encoded_file),
 		 SDATA (encoded_newname),
 		 FALSE))
-    report_file_error ("Copying file", Fcons (file, Fcons (newname, Qnil)));
+    {
+      /* CopyFile doesn't set errno when it fails.  By far the most
+	 "popular" reason is that the target is read-only.  */
+      if (GetLastError () == 5)
+	errno = EACCES;
+      else
+	errno = EPERM;
+      report_file_error ("Copying file", Fcons (file, Fcons (newname, Qnil)));
+    }
   /* CopyFile retains the timestamp by default.  */
   else if (NILP (keep_time))
     {