Update error.cpp

When compiling with g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-18) an error occurs: 
error.cpp: In member function ‘void Error::generate_error(unsigned int, std::string, std::string)’:
error.cpp:146: error: ‘exit’ was not declared in this scope
The fix is to include the #include <cstdlib> where the exit() function is decleared in the error.cpp file