Update and reorganize Python docs

doc/src/Python_error.rst

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+LAMMPS error handling in Python
+*******************************
+
+Compiling the shared library with :ref:`C++ exception support <exceptions>` provides a better error
+handling experience. Without exceptions the LAMMPS code will terminate the
+current Python process with an error message.  C++ exceptions allow capturing
+them on the C++ side and rethrowing them on the Python side. This way
+LAMMPS errors can be handled through the Python exception handling mechanism.
+
+.. warning::
+
+   Capturing a LAMMPS exception in Python can still mean that the
+   current LAMMPS process is in an illegal state and must be terminated. It is
+   advised to save your data and terminate the Python instance as quickly as
+   possible.