Also print system error message where operating system call failed.

src/pio_posixasynch.c

@@ -173,7 +173,7 @@ writePA(bFiledataPA *bfd, size_t amount)
 
   if ( iret == -1 )
     {
-      xabort ( "did not succeed writing buffer" );
+      SysError ( "did not succeed writing buffer" );
     }
   else
     xdebug ( "buffer written to %s",  bfd->name );