Error messages for saving and deleting exceptions.

Now if an exception is launched while saving or
deleting a record, the exception is let to flow
so and we catch them in higher classes and display
an error message.
The error message can be customized as described in
the properties files.
Don't change position with save error.

Now the showWarning method returns a boolean
telling us whether the save operation could
be finished properly. If there was an error,
we don't change the position.
This can't be done when closing the window
due to events limitations.