Create new LocatorNotSupportedException
Update abstract By class to use LocatorNotSupportedExceptions when class casting fails
Minor update to exception naming
Update LocatorNotSupportedException to accept a Locator as an argument and append its toString to the exception message
Add JUnit tests for stock Locator implementations
Merge pull request #17 from bdmiller3/locator_exception_handling

Throw descriptive exception when Locator not supported (fixes #6)