Throw error for when Curve is used for coil objectives (#1141)

Resolves #1020 

The given example in #1020 uses a `FourierPlanarCurve` which actually
results in a list of numpy arrays being created after going through
`tree_flatten`. This PR gives a better error message, but I'm not sure
if we can explicitly say that a `Curve` was inputted.

If there is a `Curve` object in a `CoilSet` then there are already
checks in the `CoilSet` class that throw an error.

desc/objectives/_coils.py

@@ -124,6 +124,12 @@ def _prune_coilset_tree(coilset):
 
         # get individual coils from coilset
         coils, structure = tree_flatten(coil, is_leaf=_is_single_coil)
+        for c in coils:
+            errorif(
+                not isinstance(c, _Coil),
+                TypeError,
+                f"Expected object of type Coil, got {type(c)}",
+            )
         self._num_coils = len(coils)
 
         # map grid to list of length coils

tests/test_objective_funs.py

@@ -24,7 +24,7 @@
 from desc.compute import get_transforms
 from desc.equilibrium import Equilibrium
 from desc.examples import get
-from desc.geometry import FourierRZToroidalSurface, FourierXYZCurve
+from desc.geometry import FourierPlanarCurve, FourierRZToroidalSurface, FourierXYZCurve
 from desc.grid import ConcentricGrid, LinearGrid, QuadratureGrid
 from desc.io import load
 from desc.magnetic_fields import (
@@ -869,6 +869,13 @@ def test(coil, grid=None):
         test(mixed_coils)
         test(nested_coils, grid=grid)
 
+    def test_coil_type_error(self):
+        """Tests error when objective is not passed a coil."""
+        curve = FourierPlanarCurve(r_n=2, basis="rpz")
+        obj = CoilLength(curve)
+        with pytest.raises(TypeError):
+            obj.build()
+
     @pytest.mark.unit
     def test_coil_min_distance(self):
         """Tests minimum distance between coils in a coilset."""