Throw exception instead of segfaulting on attempt to acess ROM data before ROM set.

[DoctorTeeth]

When you try to, for example, get the number of lives left before loading the ROM, the code should now throw a std::runtime_error.

Optionally, you could also write to a log and mention that this has occurred.
Seems superfluous - the user will see the exception.

Optionally, you could move that pointer check into another function to avoid repeating code.
Also seems a little unnecessary at this point, but I can see the merits.

[mgbellemare]

LGTM. Really what should be happening here is ALEInterface should allocate RomSettings & other auxiliary structures within its constructor, so as to avoid the three step process (constructor, set options, loadROM). @mhauskn : we could do this by passing a rom filename + list of options to the ALEInterface constructor. But all of this goes beyond the scope of this PR.