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

@@ -50,8 +50,12 @@ attributes and methods provided by the ``TransactionError``:
 * ``entities``: a list of all entities that caused at least one error
   in this transaction
 
-* ``errors``: a list of all ``EntityError`` objects that occurred during
-  the transaction.
+* ``errors``: a list of all ``EntityError`` objects that caused the
+  transaction to fail.
+
+* ``all_entities``, ``all_errors``: sets of all entities and errors
+  that caused either this ``TransactionError`` or any of the
+  ``EntityError`` objects it contains.
 
 * ``has_error(error_t)``: Check whether an error of type ``error_t``
   occurred during the transaction.
@@ -62,18 +66,12 @@ representation of all errors regarding the transaction in question.
 EntityError
 -----------
 
-An ``EntityError`` represents a single error that has been returned by
-the server. You might call \* ``get_entity()`` which returns the entity
-which caused the error. \* ``get_description()`` which returns a
-description of the error. \* ``get_code()`` which returns the error code
-(if specified) or 0 (if not).
-
-In fact, the ``EntityError`` class is a subclass of
-``TransactionError``. So, it inherits the ``get_entities()``. Unless
-overridden by subclasses of ``EntityError``, it return a list with only
-one item—the entity which caused this error. Similarly, unless
-overridden by subclasses, the ``get_errors()`` method returns a list
-with only one item—``[self]``.
+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.
 
 Special Errors
 ~~~~~~~~~~~~~~