DOC: Update tutorial on error handling

Summary

The error handling has been updated in the tutorial (see #52 (closed)). Also, #49 (closed) has been fixed on the way.

Focus

src/doc/tutorials/erros.rst

Test Environment

make doc on top level and look at build/doc/html/tutorials/errors.html.

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src/doc/tutorials/errors.rst

@@ -66,12 +66,21 @@ representation of all errors regarding the transaction in question.
 EntityError
 -----------
 
-It is never raised on its own but is contained in a
-``TransactionError`` (which may or may not contain other
-``EntityError`` objects) which is then raised.
-
-The ``EntityError`` is in fact a subclass of the ``TransactionError``
-class.
+An ``EntityError`` specifies the entity and the error proper that
+caused a transaction to fail. It is never raised on its own but is
+contained in a ``TransactionError`` (which may or may not contain
+other ``EntityError`` objects) which is then raised. ``EntityError``
+has several :ref:`subclasses<Special Errors>` that further specify the
+error that occurred.
+
+The ``EntityError`` class is in fact a subclass of
+``TransactionError``. Thus, it has the same methods and attributes as
+the ``TransactionError`` explained
+:ref:`above<TransactionError>`. This is important in case of an
+``EntityError`` that was caused by other faulty entities (e.g., broken
+parents or properties). In that case these problematic entities and
+errors can again be inspected by visiting the ``entities`` and
+``errors`` lists as above.
 
 Special Errors
 ~~~~~~~~~~~~~~