XML Parse Error Exception

Fixes #204 
Throws exception if the XML is not well-formed (due to user directly modifying data.xml)
Exception handling in the upper layers have not been implemented, I want to confirm that I implement the exception throwing correctly first. Most probably will do that in another PR.
Btw, this might be needed for the "Constraint-Readable"
>Constraint-Readable: The data should be stored locally and should be in a human understandable text file. The intention of this constraint is to allow advanced users to manipulate the task list by editing the data file.
might want to call it DataStoreCorruptException...