refactor: remove unused LoaderError type and associated file; streamline error handling structure

internal/app/custom_errors/loader.error.go

@@ -1,14 +0,0 @@
-package custom_errors
-
-import (
-	"git.ksdemosapps.com/kylesoda/go-migrate/internal/app/models"
-)
-
-type LoaderError struct {
-	Batch models.Batch
-	Msg   string
-}
-
-func (e *LoaderError) Error() string {
-	return e.Msg
-}

internal/app/custom_errors/types.go

@@ -14,3 +14,12 @@ type ExtractorError struct {
 func (e *ExtractorError) Error() string {
 	return e.Msg
 }
+
+type LoaderError struct {
+	Batch models.Batch
+	Msg   string
+}
+
+func (e *LoaderError) Error() string {
+	return e.Msg
+}

internal/app/etl/transformers/types.go

@@ -1 +0,0 @@
-package transformers