Improve error checking and response

[vitorsouza]

Some improvements can be done regarding general error checking in the framework. Some examples:

• ParameterImpl does many string-number conversion and calculations. It's currently returning null when something goes wrong, but it really should thrown a runtime exception, like Illegal State Exception or something.
• The EMF model defines some default values for parameters and then they are checked again in the strategies and in Qualia. Is this really necessary?
• Qualia checks the necessary information needed by each procedure in the checkApplicability() method. Some of them are then checked again when the information is used. Change that.