libopensc: invalid usage of SC_ERROR_MEMORY_FAILURE

Initiated by discussion in https://github.com/OpenSC/OpenSC/pull/134 .

SC_ERROR_MEMORY_FAILURE has to be used as a resulting code of the card related operations,
and not as result of the memory allocation problems.

New 'warning' category of SC_ERRORs introduced -- SC_WARNING_xx .
Error text and SC_ERROR associated with return codes 6300 and 6200 has been changed.