Add error handling to Exercise E3.11. If the user does not enter a number when expected, or provides an invalid unit for the altitude, print an error message and end the program.

Data from Exercise E3.11. 

The boiling point of water drops by about one degree centigrade for every 300 meters (or 1,000 feet) of altitude. Improve the program of Exercise E3.10 to allow the user to supply the altitude in meters or feet.