Continued:

Continued:
- normalized exceptions, like when an entity has already been added (with same
  title here) or when it was not found. Already added exceptions always indicate
  problems with calling methods by "not-found" exceptions may be triggered by
  invalid "show" links or such kinds (when a non-existing id number for example
  was provided by parameter).
- added constructor with id number (common way)

Signed-off-by: Roland Häder <roland@mxchange.org>