Offer advice for association typo

Closes https://github.com/thoughtbot/factory_bot/issues/1227

In this code we are passing an implicit declaration `user`, rather than
the symbol `:user`:

```rb
factory :post do
  author factory: user
end
```

In https://github.com/thoughtbot/factory_bot/issues/1227 we improved the
error message from:

```rb
undefined method 'name' for :post:Symbol
```

to:

```rb
Trait not registered: user
```

But this still doesn't make it totally obvious what the error was.
Why are we trying to register a user trait?

With this PR we update the error message to:

```rb
Association 'author' received an invalid factory argument.
Did you mean? 'factory: :user'
```

lib/factory_bot/declaration/association.rb

@@ -22,9 +22,20 @@ module FactoryBot
       private
 
       def build
+        ensure_factory_is_not_a_declaration!
+
         factory_name = @overrides[:factory] || name
         [Attribute::Association.new(name, factory_name, [@traits, @overrides.except(:factory)].flatten)]
       end
+
+      def ensure_factory_is_not_a_declaration!
+        if @overrides[:factory].is_a?(Declaration)
+          raise ArgumentError.new(<<~MSG)
+            Association '#{name}' received an invalid factory argument.
+            Did you mean? 'factory: :#{@overrides[:factory].name}'
+          MSG
+        end
+      end
     end
   end
 end

spec/acceptance/associations_spec.rb

@@ -9,8 +9,11 @@ describe "associations" do
         end
       end
 
-      expect { FactoryBot.build(:post) }.
-        to raise_error(KeyError, "Trait not registered: \"user\"")
+      expect { FactoryBot.build(:post) }.to raise_error(
+        ArgumentError,
+        "Association 'author' received an invalid factory argument.\n" \
+        "Did you mean? 'factory: :user'\n",
+      )
     end
   end
 end