CLI error handling and reporting

[rubydesign]

The ruby layer has very very little error checking, and even less error reporting. Since we have a cli and new users, error handling and messages would be great.

This starts by writing tests with ruby functionality that is not handled. Then improving the error handling. Any ruby can be used (ie try to find the limits of the compiler), but here are some starting points:

• next and break in while loops
• begin and rescue statements (maybe warning about using raise)
• case, when
• nested scopes
• global variables