class Error(Exception):
    def __str__(self):
        return "%s: %s" %(self.__class__.__name__, self.args[0])
class UnsupportedInterpreter(Error):
    "signals an unsupported Interpreter"
class InterpreterNotFound(Error):
    "signals that an interpreter could not be found"
class InvocationError(Error):
    """ an error while invoking a script. """
class MissingFile(Error):
    """ an error while invoking a script. """
class MissingDirectory(Error):
    """ a directory did not exist. """
class MissingDependency(Error):
    """ a dependency could not be found or determined. """