Added new error class

The interface EnumError class was added to manage the errors produced
in systems that use a code system for error reporting.
Added tests for errors

CallError: A new test was added to implement the copy constructor.
EnumError: thest were added to implement the error class.
Implemented the the code() method
Implemented tests for EnumError

Additionally, a "problem" was found while implementing tests. The
EnumError and the CallError tests are not well done. There are
redundant tests that have to be maintained. This should be fixed...
Added Doxygen documentation

Some documentation about errors was added to the code.