feat: add generic lnurl error response handler (#2638)

* feat: add generic lnurl error response handler

this is used multiple times in extensions to safeguard `views_lnurl.py`
endpoint to not return a wrong lnurl error response.

you use it by just setting following on your lnurl router/view

```
withdraw_ext_lnurl = APIRouter(prefix="/api/v1/lnurl")
withdraw_ext_lnurl.route_class = LNURLErrorResponseHandler
```

lnbits/exceptions.py

@@ -8,11 +8,21 @@ from fastapi.exceptions import RequestValidationError
 from fastapi.responses import JSONResponse, RedirectResponse, Response
 from loguru import logger
 
-from lnbits.core.services import InvoiceError, PaymentError
-
 from .helpers import template_renderer
 
 
+class PaymentError(Exception):
+    def __init__(self, message: str, status: str = "pending"):
+        self.message = message
+        self.status = status
+
+
+class InvoiceError(Exception):
+    def __init__(self, message: str, status: str = "pending"):
+        self.message = message
+        self.status = status
+
+
 def register_exception_handlers(app: FastAPI):
     register_exception_handler(app)
     register_request_validation_exception_handler(app)