Refactored ColorIndexConversionException to exist in its own class file
rather than PatternMaker, and created method
ColorIndexConversionException.makeMessage(int) to receive named constant
'TOO_LOW' or 'TOO_HIGH' or 'IS_NULL' and generate an error message.
Will refactor makeMessage(int) into a factory method to simplify name
space at next commit.

Signed-off-by: Henry Dwyer <darkstar43.0@gmail.com>