Derived exception class for more practical error reporting (Feature #124)

I suggest a tiny but practical enhancement, deriving cv::Exception from std::exception so that error reporting will be more practical. It will not be necessary to write additional handler for opencv every time. I put my implementation below.

CV_EXPORTS string format( const char* fmt, ... );

class CV_EXPORTS Exception : public std::exception {
public:
Exception() { code = 0; line = 0; }
Exception(int _code, const string& _err, const string& _func, const string& _file, int _line)
: code(_code), err(_err), func(_func), file(_file), line(_line) {}
Exception(const Exception& exc)
: code(exc.code), err(exc.err), func(exc.func), file(exc.file), line(exc.line) {}
Exception& operator = (const Exception& exc) {
if( this != &exc ) {
code = exc.code; err = exc.err; func = exc.func; file = exc.file; line = exc.line;
}
return *this;
}

virtual const char *what() const throw()
{
        String str = format("Code: %d\nError: %s\nFunction: %s\nFile: %s\nLine: %d\n",
                             code, err.c_str(), func.c_str(), file.c_str(), line);
        char *_msg = const_cast<char *>(msg);
        strncpy(_msg, str.c_str(), max_strlen);
        _msg[max_strlen-1] = '\0';
        return msg;
    }

static const size_t max_strlen = 1024;
    char msg[max_strlen];

int code;
    string err;
    string func;
    string file;
    int line;
};