Better error messages #2

Need to wrap the execute/run call procedure in a begin/rescue clause and only output one line messages by default, The $DEBUG variable could be used to let errors through the rescue clause.

Also might provide an Executable::ValidationError or something like that, and a help method to make it easier to check the validity of option values. Then again, there are probably some really good validation libraries out there that can already be integrated. So we'll see.