Fix #384

show_source now shows the following error messages:
* for empty string: ""Cannot get source of an empty string"
* for a string that matches a built-in: "Cannot access source of <built-in function X>"
* for a string that doesn't match any object: "Cannot get source: X is not defined"
* if the object doesn't have source code: "No source code found for X"
* else: the error message from the original inspect.getsource() error