[python][cmake] For DUNE_PYTHON_USE_VENV=FALSE exit FATAL_ERROR if attempt to...

[python][cmake] For DUNE_PYTHON_USE_VENV=FALSE exit FATAL_ERROR if attempt to call dune_python_configure_dependencies

cmake/modules/DunePythonInstallPackage.cmake

@@ -161,7 +161,7 @@ include_guard(GLOBAL)
 
 function(dune_python_configure_dependencies)
   if(NOT DUNE_PYTHON_USE_VENV)
-    return()
+    message(FATAL_ERROR "Tried to install Python dependencies when the use of virtual environments has been disabled via DUNE_PYTHON_USE_VENV=FALSE")
   endif()
 
   # Parse Arguments