Handle missing README in create repos API (#23387) (#23509)

Backport #23387 
Close #22934

In `/user/repos` API (and other APIs related to creating repos), user
can specify a readme template for auto init. At present, if the
specified template does not exist, a `500` will be returned . This PR
improved the logic and will return a `400` instead of `500`.

templates/swagger/v1_json.tmpl

@@ -586,6 +586,9 @@
           "201": {
             "$ref": "#/responses/Repository"
           },
+          "400": {
+            "$ref": "#/responses/error"
+          },
           "403": {
             "$ref": "#/responses/forbidden"
           },
@@ -1772,6 +1775,9 @@
           "201": {
             "$ref": "#/responses/Repository"
           },
+          "400": {
+            "$ref": "#/responses/error"
+          },
           "403": {
             "$ref": "#/responses/forbidden"
           },
@@ -12502,6 +12508,9 @@
           "201": {
             "$ref": "#/responses/Repository"
           },
+          "400": {
+            "$ref": "#/responses/error"
+          },
           "409": {
             "description": "The repository with the same name already exists."
           },