api: return 500 instead of 404 in case of non-existing items (#2582)

This is needed to differentiate between calls to non-existent andpoints
and calls to valid endpoints, but with an ID pointing to a non-existing
item, and to return JSON in the latter case.

internal/core/path_manager.go

@@ -383,7 +383,7 @@ func (pm *pathManager) doAPIPathsList(req pathAPIPathsListReq) {
 func (pm *pathManager) doAPIPathsGet(req pathAPIPathsGetReq) {
 	path, ok := pm.paths[req.name]
 	if !ok {
-		req.res <- pathAPIPathsGetRes{err: errAPINotFound}
+		req.res <- pathAPIPathsGetRes{err: fmt.Errorf("path not found")}
 		return
 	}