Fix handling of migration errors (#12928)

* Fix handling of migration errors

The migration type selection screen PR did not correctly handle errors
and any user input error on the migration page would simply redirect
back to the selection page. This meant that the error would simply be
lost and the user would be none the wiser as to what happened.

Signed-off-by: Andrew Thornton <art27@cantab.net>

* make gen-swagger

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

templates/swagger/v1_json.tmpl

@@ -13231,6 +13231,12 @@
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"
     },
+    "GitServiceType": {
+      "description": "GitServiceType represents a git service",
+      "type": "integer",
+      "format": "int64",
+      "x-go-package": "code.gitea.io/gitea/modules/structs"
+    },
     "GitTreeResponse": {
       "description": "GitTreeResponse returns a git tree",
       "type": "object",
@@ -13658,9 +13664,7 @@
           "x-go-name": "RepoName"
         },
         "service": {
-          "type": "integer",
-          "format": "int64",
-          "x-go-name": "Service"
+          "$ref": "#/definitions/GitServiceType"
         },
         "uid": {
           "type": "integer",