Allow to mark API execution as failed (and don't charge them)

Allow services to signal that API execution has failed, which should free up the payment token and allow it to be used for another API call.

- If the `X-Charge` response header is set to `yes`, consider the request a success and mark the payment token as spent. If the `X-Charge` header is set to any other value (normally `no`), consider the request a failure and mark the payment token as usable.

- If the `X-Charge` response header is not specified,

- If the `--explicit-charge` config option is set, consider the request a failure.

- If the `--explicit-charge` config option is not set, consider the request a success if the status code is `2xx` or `3xx`, otherwise consider it a failure.

Freeing up the payment token means removing it from the `spent` table, which means we should log the execution error to a separate (new) table.