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

 
 Error Handling
---------------
+==============
 
-HeartDBException
-~~~~~~~~~~~~~~~~
+In case of erroneous transactions, connection problems and a lot of
+other cases, PyCaosDB may raise specific errors in order to pinpoint
+the problem as precisely as possible. Some of these errors a
+representations of errors in the CaosDB server, others stem from
+problems that occurred on the client side.
+
+The errors and exceptions are ordered hierarchically form the most
+general exceptions to specific transaction or connection problems. The
+most important error types and the hierarchy will be explained in the
+following. For more information on specific error types, see also the
+`source code
+<https://docs.indiscale.com/caosdb-pylib/_apidoc/caosdb.html#module-caosdb.exceptions>`_.
+
+.. note::
+
+   From PyCaosDB 0.5 and newer, the error handling has changed
+   significantly. New error classes have been introduced and the
+   behavior of ``TransactionError`` and ``EntityError`` has been
+   re-worked. In the following, only the "new" errors are
+   discussed. Please refer to the documentation of PyCaosDB 0.4.1 and
+   earlier for the old error handling.
+
+CaosDBException
+----------------
+
+This is the most generic exception and all other error classes inherit
+from this one. Because of its generality, it doesn't tell you much
+except that some component of PyCaosDB raised an exception. If you
+want to catch all possible CaosDB errors, this is the class to use.
 
 TransactionError
-~~~~~~~~~~~~~~~~
+----------------
 
 Every transaction (calling ``insert``, ``update``, ``retrieve``, or
 ``delete`` on a container or an entity) may finish with errors. They
 indicate, for instance, that an entity does not exist or that you need
 to specify a data type for your property and much more. If and only if
 one or more errors occur during a transaction a ``TransactionError``
-will be raised by the transaction method. The ``TransactionError`` class
-is a container for all errors which occur during a transaction. It can
-help you to find the crucial problems with your transaction by two
-important methods: \* ``get_errors()`` which returns a list of instances
-of ``EntityError``. \* ``get_entities()`` which returns a list of
-entities in the transaction container which are erroneous.
-
-Additionally, ``print(transaction_error`` prints a tree-like
+will be raised by the transaction method. The ``TransactionError``
+class is a container for all errors which occur during a
+transaction. It usually contains one or more :ref:`entity
+errors<EntityError>` which you can inspect in order to learn why the
+transaction failed. For this inspection, there are some helpful
+attributes and methods provided by the ``TransactionError``:
+
+* ``entities``: a set of all entities that caused at least one error
+  in this transaction
+
+* ``errors``: a set of all ``EntityError`` objects that occurred during
+  the transaction.
+
+* ``has_error(error_t)``: Check whether an error of type ``error_t``
+  occurred during the transaction.
+
+Additionally, ``print(transaction_error)`` prints a tree-like
 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