raise error for bad connection types, add unit test

gmso/core/topology.py

@@ -1433,6 +1433,12 @@ def iter_connections_by_site(self, site, connections=None):
             connections = ["bonds", "angles", "dihedrals", "impropers"]
         else:
             connections = set([option.lower() for option in connections])
+            for option in connections:
+                if option not in ["bonds", "angles", "dihedrals", "impropers"]:
+                    raise ValueError(
+                        "Valid connection types are limited to: "
+                        '"bonds", "angles", "dihedrals", "impropers"'
+                    )
         for conn_str in connections:
             for conn in getattr(self, conn_str):
                 if site in conn.connection_members:

gmso/tests/test_topology.py

@@ -951,6 +951,15 @@ def test_iter_connections_by_site_none(self, ethane):
         ):
             assert site in conn.connection_members
 
+    def test_iter_connections_by_site_bad_param(self, ethane):
+        ethane.identify_connections()
+        site = ethane.sites[0]
+        with pytest.raises(ValueError):
+            for conn in ethane.iter_connections_by_site(
+                site=site, connections=["bond"]
+            ):
+                pass
+
     def test_write_forcefield(self, typed_water_system):
         forcefield = typed_water_system.get_forcefield()
         assert "opls_111" in forcefield.atom_types