Fix pattern matching error when creating uniqueness index (#112)

* Fix pattern matching error when creating uniqueness index

* Ensure returning the expected tuple for uniqueness validations

* Remove unnecessary string interpolation

Author: tiagopog

lib/mongo_ecto.ex

@@ -650,8 +650,10 @@ defmodule Mongo.Ecto do
 
     query = %WriteQuery{coll: "system.indexes", command: index}
 
-    {:ok, _} = Connection.insert(repo, query, opts)
-    :ok
+    case Connection.insert(repo, query, opts) do
+      {:ok, _} -> :ok
+      {:invalid, [unique: index]} -> raise Connection.format_constraint_error(index)
+    end
   end
 
   def execute_ddl(repo, {:drop, %Index{name: name, table: coll}}, opts) do

lib/mongo_ecto/connection.ex

@@ -189,6 +189,10 @@ defmodule Mongo.Ecto.Connection do
     end
   end
 
+  def format_constraint_error(index) do
+    %Mongo.Error{message: "ERROR (11000): could not create unique index \"#{index}\" due to duplicated entry"}
+  end
+
   defp format_query(%Query{action: :command}, [command]) do
     ["COMMAND " | inspect(command)]
   end