Improve message for duplicate name on schema

Author: josevalim

lib/ecto/schema.ex

@@ -2160,7 +2160,7 @@ defmodule Ecto.Schema do
     fields = Module.get_attribute(mod, :struct_fields)
 
     if List.keyfind(fields, name, 0) do
-      raise ArgumentError, "field/association #{inspect name} is already set on schema"
+      raise ArgumentError, "field/association #{inspect name} already exists on schema, you must either remove the duplication or choose a different name"
     end
 
     Module.put_attribute(mod, :struct_fields, {name, assoc})

test/ecto/schema_test.exs

@@ -384,7 +384,7 @@ defmodule Ecto.SchemaTest do
   ## Errors
 
   test "field name clash" do
-    assert_raise ArgumentError, "field/association :name is already set on schema", fn ->
+    assert_raise ArgumentError, ~r"field/association :name already exists on schema", fn ->
       defmodule SchemaFieldNameClash do
         use Ecto.Schema