show error message removing material that is used in material sets

previously it wasn't showing anything when you would try to remove a material

src/blenderbim/blenderbim/bim/module/material/operator.py

@@ -151,7 +151,10 @@ class RemoveMaterial(bpy.types.Operator, tool.Ifc.Operator):
     material: bpy.props.IntProperty()
 
     def _execute(self, context):
-        core.remove_material(tool.Ifc, tool.Material, tool.Style, material=tool.Ifc.get().by_id(self.material))
+        res = core.remove_material(tool.Ifc, tool.Material, tool.Style, material=tool.Ifc.get().by_id(self.material))
+        if not res:
+            self.report({"ERROR"}, "Material is used in material sets and cannot be removed.")
+            return {"CANCELLED"}
 
 
 class RemoveMaterialSet(bpy.types.Operator, tool.Ifc.Operator):

src/blenderbim/blenderbim/core/material.py

@@ -43,9 +43,11 @@ def add_material_set(ifc, material, set_type=None):
     return ifc_material
 
 
-def remove_material(ifc, material_tool, style, material=None):
+def remove_material(ifc, material_tool, style, material=None) -> bool:
+    """returns True after deleting False,\n
+    returns False if material used in material sets and cannot be removed"""
     if material_tool.is_material_used_in_sets(material):
-        return
+        return False
     obj = ifc.get_object(material)
     ifc.unlink(element=material)
     ifc.run("material.remove_material", material=material)
@@ -53,6 +55,7 @@ def remove_material(ifc, material_tool, style, material=None):
         material_tool.delete_object(obj)
     if material_tool.is_editing_materials():
         material_tool.import_material_definitions(material_tool.get_active_material_type())
+    return True
 
 
 def remove_material_set(ifc, material_tool, material=None):