Map Transistor SDK error codes to human-readable string keys

Currently Marco passes through raw numeric error codes from the Transistor Background Geolocation SDK (e.g., 408 for location timeout).

These should be mapped to descriptive string keys for better DX:

| SDK Code | Proposed Key |
|----------|--------------|
| 0 | LOCATION_UNKNOWN |
| 1 | PERMISSION_DENIED |
| 2 | NETWORK_ERROR |
| 408 | LOCATION_TIMEOUT |

Reference: https://github.com/transistorsoft/cordova-background-geolocation-lt/wiki/Location-Error-Codes