Missing python-bindings requirements should not lead to a FATAL_ERROR in CMake

Summary

If Python is not found, the python bindings are not activated. If, however, during the setup of the python bindings some python requirement cannot be fulfilled (and also not automatically installed), CMake raises a hard error. This happened, e.g., in a Linux Mint 19 system (based on Ubuntu 18.04) with an unusable pip. Warning are shown and the hard error message indicates how to solve the problem, but for those who do not want to use any python bindings this is annoying, because it is absolutely not needed for the c++ library.

Proposed solution: transform FATA_ERRORs in CMake into WARNING and deactivate the python bindings instead. Maybe collect all these problems in a variable DUNE_PYTHON_BINDINGS_REQUIREMENTS_FOUND that is set to FALSE in any of the current FATAL_ERROR cases.

Related to #275.

Edit

The issue is not a hard error during the installation of the requirements for the python bindings. A problem there leads to a warning only. The issue is a hard error when setting up the internal venv and pip is not available.