Retrieve and log perror value for low level POSIX file methods.

The current error handling code in IOM::thr_popper() (JFYI the class name is not precise) can't tell what actually goes wrong when pread fails, e.g. it is impossible to understand why GlusterFS fails reading compression headers in updateptrs() so it is unclear how to manage with the problem.
It would be helpful to print out perror value in case of an error, namely for the case when updateptrs returns -4.