Provide constant with minimum string size for error messages
wrtd_error_message() documentation says that the err_message buffer should be at least 256 long.
This value should be provided as a constant (eg WRTD_MIN_ERR_MESSAGE_BUFFER_SIZE) so that applications can use this instead of hard-coding the 256 value (which could change in the future). Something similar to what we do with the WRTD_LOG_ENTRY_SIZE