refactor(BA-2753): Move error classes to its own module

Author: hhoikoo

src/ai/backend/agent/errors/__init__.py

@@ -0,0 +1,17 @@
+from ai.backend.common.exception import (
+    BackendAIError,
+    ErrorCode,
+    ErrorDetail,
+    ErrorDomain,
+    ErrorOperation,
+)
+
+
+class AgentIdNotFoundError(BackendAIError):
+    @classmethod
+    def error_code(cls) -> ErrorCode:
+        return ErrorCode(
+            domain=ErrorDomain.AGENT,
+            operation=ErrorOperation.ACCESS,
+            error_detail=ErrorDetail.NOT_FOUND,
+        )