Cleanup: Standardize validation messages
(Imported from Google Code)
Issue #208 cleaned up the validation text in every class' header javadoc. Need to update the class-thrown errors as well.
e.g. "attribute is required" vs. "attribute must exist".
Standardize wording to match the rules in the header comment.
Standarize how fields are mentioned "createDate" vs "creation date" vs "ism:createDate".
Complete in Rev 843.