Currently `removeSplitter()` function attempts to merge coplanar faces
by using `ModelRefine::FaceUniter`, which internally uses
`BRepBuilderAPI_Sewing`. This can fail in several scenarios, like
inability to produce a valid shell, failing in face unification due
to geometry incompatibilities, or when we have tolerance issue in the
reconstructed geometry from glTF triangulation that prevent us from face
merging.
That leads to the exception, which we are not handling correctly where
it's being handled the same way as this patch introduces in other parts
of FreeCAD.
So, this patch adds a try-catch block around `removeSplitter()` to
gracefully handle `Standard_Failure` exception and return the sewn shape
instead of crashing. Imported model will contain more individual faces
than necessary (since coplanar won't be merged), resulting in a more
complex topology in the places of those fails, but geometry and visual
appearance will be preserved.
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