Added exceptions and exception handling

[aaron97neu]

In many languages, Python allows for throwing and catching exceptions (as well as defining your own). Implementing this into LIMO would be extremely useful to allow for better debugging for the end user. Since we are teaching beginning programmers, the ability to immediately halt the program and display a relevant and informative message about what they did wrong (in the code example from #17, something like "Direction was not valid. must be a bearing in degrees, or N/S/E/W or Left/Right") would help them fix their programs much faster, as well as avoid the convoluted method of returning invalid ("-1" or "null") values and hoping the calling method responds appropriately

[aaron97neu]

It appears that on get_road_names(), geocode_intersection(), and geocode_address() all raise one exception each. This implies someone at some point in time started this. This might be a good starting point