Fixed type mismache error on amd64

CppUnit assertion was failing because it expects two objects form the
same data type.
Improved comparisson on DataError

Some tests were added to improve the comparisson methods of the
DataError class.
Outlined the interface for error classes

The desired interface for the classes CallError and ObjectError was
included to have a clear idea of was is going to be implemented.
Added test for CallError

Test to implement the CallError class were added.