Add get_error method to client for standardized error retrieval

- Implemented get_error() method to fetch job error messages from Redis
- Mirrors get_result() pattern for consistency
- Used by supervisor to retrieve job errors without manual Redis queries
- Cleanup: removed old runner_osis directory

src/client.rs

@@ -312,6 +312,21 @@ impl Client {
         Ok(result)
     }
 
+    /// Get job result from Redis
+    pub async fn get_error(
+        &self,
+        job_id: &str,
+    ) -> Result<Option<String>, JobError> {
+        let job_key = self.job_key(job_id);
+        let mut conn = self.redis_client
+            .get_multiplexed_async_connection()
+            .await
+            .map_err(|e| JobError::Redis(e))?;
+        let result: Option<String> = conn.hget(&job_key, "error").await
+            .map_err(|e| JobError::Redis(e))?;
+        Ok(result)
+    }
+
     /// Get a job ID from the work queue (blocking pop)
     pub async fn get_job_id(&self, queue_key: &str) -> Result<Option<String>, JobError> {
         let mut conn = self.redis_client