Patches Load: Exception Handling

Describe the bug Loading a non-existent particle patch should not segfault but throw (in Python).

Generally, try to derive and document a high-level workflow to deal with particle patches for scripts and frameworks such as ParaView.

To Reproduce https://github.com/ComputationalRadiationPhysics/particle_reduction/issues/8

Expected behavior Throw an exception and add simple probing functionality or high-level iterators.

Software Environment

• version of openPMD-api: 0.11.0-alpha
• installed openPMD-api via: all
• operating system: Ubuntu Linux 18.04 / all
• machine: local laptops / all
• name and version of Python implementation: CPython 3.7/3.8
• version of HDF5: N/A
• version of ADIOS1: N/A
• version of ADIOS2: N/A
• name and version of MPI: N/A

Additional context Related to report in https://github.com/ComputationalRadiationPhysics/particle_reduction/issues/8