Added the project specific exception class: HmException (see issue #2).
Using the HmException in the Account and AccountManager classes.
Using the HmException class in the CategoryManager.
Using the HmException in the PayeeManager.
Using the HmException in the ValueCalculator.
Using the HmException in the HmReader.
Using the HmException in the Category.
Merging with the exception branch. Fixes #2.