memcached_strerror() takes a "memcached_return_t" value and returns a string
describing the error.
This string must not be modified by the application.
"memcached_return_t" values are returned from nearly all libmemcached(3) functions.
"memcached_return_t" values are of an enum type so that you can set up responses
with switch/case and know that you are capturing all possible return values.
RETURN
memcached_strerror() returns a string describing a "memcached_return_t" value.