Fail gracefully

[phadej]

Currently we don't fail gracefully, thus e.g. when we run tests we get situations like

golden
  bitfields
    treediff: golden: bit-fields not supported yet
CallStack (from HasCallStack):
  error, called at src/HsBindgen/C/Fold/Type.hs:391:47 in hs-bindgen-0.1.0-inplace:HsBindgen.C.Fold.Type

There's two part, the tree-diff probably should catch exceptions and fail more gracefully (report test as failed, not abort everything);
But also we should fail more gracefully to begin with, I'm sure there will be unsupported features for a long time.