Add error message for assertTrue in solidity tests

[ehildenb]

@gbalabasquer you'll also need to change mcd-pyk.py. Specifically, function makeSolidityCall and buildAsserts.

I would just keep a global counter in the python file, and have each assert print out something like "assert NNN" where N is the global counter number of each time it makes an assert. This will make it easier to see which assert failed.

I can do it instead if you like.

[gbalabasquer]

@ehildenb ahh ok, if you can easily do it please go ahead, it was a long time since I've used python so you will probably do it much faster