Add missing check to permissionSchemeService.Get

There was missing a check for requests with invalid ID.
Now it returns an error instead of an empty permissionScheme.

permissionscheme.go

@@ -61,6 +61,9 @@ func (s *PermissionSchemeService) Get(schemeID int) (*PermissionScheme, *Respons
 		jerr := NewJiraError(resp, err)
 		return nil, resp, jerr
 	}
+	if ps.Self == "" {
+		return nil, resp, fmt.Errorf("No permissionscheme with ID %d found", schemeID)
+	}
 
 	return ps, resp, nil
 }