Balancing Reaction fails if elements not included on both sides.

[Pabreetzio]

Currently if the user creates a reaction that does not have an example element on both sides of the equation then the balancing blows up. This should gracefully provide the user with feedback instead of failing.

[WillTheProgrammer]

So- what I'm thinking is that the balance button should be disabled unless the user has matching elements in both the product side and reactant side.... playing around with that now.