Added API for library error code

[jonghee-son]

Added API function to retrieve library error code.

In line 158 to 161 in Dynamixel2Arduino.cpp

int Dynamixel2Arduino::getLibErrCode()
{
  return getLastLibErrCode();
}

In line 98 to 107 in Dynamixel2Arduino.h

/**
 * @brief API for getting library error code.
 * @code
 * const int DXL_DIR_PIN = 2;
 * Dynamixel2Arduino dxl(Serial1, DXL_DIR_PIN);
 * Serial.print(dxl.getErr());
 * @endcode
 * @return It returns error code from library.
 */
int getLibErrCode();