[Hello World] Added Hello World to ImportError Message Extraction

[BethanyG]

Decided that we should do for Hello World what we did for Lasagna (see #110 for some details).

Added hello_world to the function starting on line 140:

    def pytest_exception_interact(self, node, call, report):
        """
        Catch the last exception handled in case the test run itself errors.
        """
        if report.outcome == "failed":
            excinfo = call.excinfo
            err = excinfo.getrepr(style="no", abspath=False)

            # trim off full traceback for first two exercises to be friendlier and clearer
            if ('lasagna' in node.name or 'hello_world' in node.name) and 'ImportError' in str(err.chain[0]):
                trace = err.chain[-2][0]
            else:
                trace = err.chain[-1][0]

            crash = err.chain[0][1]
            self.last_err = self._make_message(trace, crash)

Now they will see the following in the UI (once the test file changes in the content repo are merged):

ImportError: 
   
MISSING FUNCTION --> In your 'hello_world.py' file, we can not find or import the function named hello().
The tests for this first exercise expect a function that returns the string "Hello, World!"
 
Did you use print("Hello, World!") instead?

Also added a test for this exercise/scenario.

[BethanyG]

Many thanks!