Improve error message.

src/pio_posixnonb.c

@@ -50,8 +50,11 @@ initBFiledataP(struct fileFunnelStdio *bfp,
       }
   }
 
-  if (( bfp->fp = fopen ( filename, "w" )) == NULL )
-    xabort("Failed to open %s", bfp->name);
+  if (( bfp->fp = fopen ( filename, "w" )) == NULL ) {
+    int fopen_errno = errno;
+    xabort("Failed to open %s\nerrno=%d: %s", bfp->name, fopen_errno,
+           strerror(fopen_errno));
+  }
 
   struct dBuffer *db = bfp->db = Malloc(sizeof (*db) * nc);
   for (size_t i = 0; i < nc; ++i)