Limited fix on svn proc_open pipe handling

Created by: obmsch

This is is just a reordering on the pipes and the rational about it.

Due to limitations on the buffering of pipes and therefore possible resulting deadlocks, the order the pipes are read is crucial. The reordering (STDOUT before STDERR) will not(!) prevent a deadlock, if the output of the process to STDERR exceeds the max buffered size on STDERR.

Better handle the unlikely error, than get stuck on the goods.