lnurl: better handle unimplemented lnurl subtypes

fixes https://github.com/spesmilo/electrum/issues/7971

electrum/gui/kivy/uix/screens.py

@@ -237,9 +237,12 @@ class SendScreen(CScreen, Logger):
     def set_lnurl6(self, lnurl: str):
         url = decode_lnurl(lnurl)
         domain = urlparse(url).netloc
-        # FIXME network request blocking GUI thread:
-        lnurl_data = Network.run_from_another_thread(request_lnurl(url))
-        if not lnurl_data:
+        try:
+            # FIXME network request blocking GUI thread:
+            lnurl_data = Network.run_from_another_thread(request_lnurl(url))
+        except LNURLError as e:
+            self.app.show_error(f"LNURL request encountered error: {e}")
+            self.do_clear()
             return
         self.lnurl_data = lnurl_data
         self.address = "invoice from lnurl"

electrum/lnurl.py

@@ -66,11 +66,11 @@ async def _request_lnurl(url: str) -> dict:
     return response
 
 
-async def request_lnurl(url: str) -> Optional[LNURL6Data]:
+async def request_lnurl(url: str) -> LNURL6Data:
     lnurl_dict = await _request_lnurl(url)
     tag = lnurl_dict.get('tag')
     if tag != 'payRequest':  # only LNURL6 is handled atm
-        return None
+        raise LNURLError(f"Unknown subtype of lnurl. tag={tag}")
     metadata = lnurl_dict.get('metadata')
     metadata_plaintext = ""
     for m in metadata: