Error message for non-existing backends

[gronerl]

When compiling a stencil with the backend parameter set to an invalid string value currently raises

Traceback (most recent call last):
  File "/home/gronerl/gt4py/tmp.py", line 5, in <module>
    def test_stencil(out_storage: gtscript.Field[int]):
  File "/home/gronerl/gt4py/src/gt4py/gtscript.py", line 459, in _decorator
    StencilBuilder(
  File "/home/gronerl/gt4py/src/gt4py/stencil_builder.py", line 73, in __init__
    self.backend: "BackendType" = backend(self)
TypeError: 'NoneType' object is not callable

And should be a ValueError caught early on.

[jdahm]

I believe that #979 indirectly addresses this because it has to be type correct. I'll add a proper exception with a good error message in that case.