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