ptxed: print file handling errors

All errors with file operations result in -pte_bad_config.  This makes
it impossible to see what exactly went wrong.

Add error messages.

Change-Id: Ic02016ab5558845e9f12de8797616453e0f6f2d1
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>