Report errors correctly on failure to read NTFS usage (#57)

GParted uses ntfsinfo to read the NTFS file system usage.  However when
ntfsinfo fails with a non-zero exit status GParted reports stdout twice,
rather than stdout and stderr.  Correct this.

Closes #57 - NTFS Resize results in Partition Information Warning on
             Refresh

src/ntfs.cc

@@ -112,7 +112,7 @@ void ntfs::set_used_sectors( Partition & partition )
 		if (! output.empty())
 			partition.push_back_message(output);
 		if (! error.empty())
-			partition.push_back_message(output);
+			partition.push_back_message(error);
 		return;
 	}