Put the repo name in these errors

This is helpful if the application has more than one repo (eg read
replicas). For example:

Before: "** (DBConnection.ConnectionError) connection is closed because
of an error, disconnect or timeout"

After: "** (DBConnection.ConnectionError) MyApp.Repo connection is
closed because of an error, disconnect or timeout"

Author: nathanl

lib/db_connection/holder.ex

@@ -135,25 +135,43 @@ defmodule DBConnection.Holder do
       :ets.lookup(holder, :conn)
     rescue
       ArgumentError ->
-        msg = "connection is closed because of an error, disconnect or timeout"
+        msg =
+          maybe_prefix_repo(
+            "connection is closed because of an error, disconnect or timeout",
+            opts
+          )
+
         {:disconnect, DBConnection.ConnectionError.exception(msg), _state = :unused}
     else
       [conn(lock: conn_lock)] when conn_lock != lock ->
-        raise "an outdated connection has been given to DBConnection on #{fun}/#{length(args) + 2}"
+        raise maybe_prefix_repo(
+                "an outdated connection has been given to DBConnection on #{fun}/#{length(args) + 2}",
+                opts,
+                ":"
+              )
 
       [conn(status: :error)] ->
-        msg = "connection is closed because of an error, disconnect or timeout"
+        msg =
+          maybe_prefix_repo(
+            "connection is closed because of an error, disconnect or timeout",
+            opts
+          )
+
         {:disconnect, DBConnection.ConnectionError.exception(msg), _state = :unused}
 
       [conn(status: :aborted)] when type != :cleanup ->
-        msg = "transaction rolling back"
+        msg = maybe_prefix_repo("transaction rolling back", opts)
         {:disconnect, DBConnection.ConnectionError.exception(msg), _state = :unused}
 
       [conn(module: module, state: state)] ->
         holder_apply(holder, module, fun, args ++ [opts, state])
     end
   end
 
+  defp maybe_prefix_repo(msg, opts, separator \\ "") do
+    if opts[:repo], do: "#{inspect(opts[:repo])} " <> separator <> msg, else: msg
+  end
+
   ## Pool state helpers API (invoked by callers)
 
   @spec put_state(pool_ref :: any, term) :: :ok