Fix error message when there are no credits

[Gisson]

Tried to create a WSG without enough credits and got

│ Error: SingleStore API client returned status code Forbidden
│
│   with singlestoredb_workspace_group.example,
│   on main.tf line 17, in resource "singlestoredb_workspace_group" "example":
│   17: resource "singlestoredb_workspace_group" "example" {
│
│ An unsuccessful status code occurred when calling SingleStore API. Ensure a valid API key is created at https://portal.singlestore.com/organizations/org-id/api-keys and it's provided in one of the following
│ ways:
│ 1. Directly set as the 'api_key' attribute in the provider configuration.
│ 2. Stored in a file with its absolute path set in the 'api_key_path' attribute.
│ 3. Set as the 'SINGLESTOREDB_API_KEY' environment variable.If the error is not clear, please report the issue https://www.singlestore.com/support/.
│
│ SingleStore client response body: error creating workspace group (group): no active plan found for project

This is misleading as there is no problem with the API key but with credit balance. Error message should reflect that