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 erroneus transactions, connection problems and a lot of
+other cases, PyCaosDB may raise specific errors in order to pinpoint
+the problem as precisly as possible. Some of these errors a
+representations of errors in the CaosDB server, otheres stem from
+problems that occurred on the client side.
+
+The errors and exceptions are ordered hierarichally 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
@@ -24,7 +51,7 @@ 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