dev: harden resolver bootstrap and simplify update settings

addon/resolveurl_backend.py

@@ -23,6 +23,7 @@ def resolve(url: str) -> Optional[str]:
     try:
         import resolveurl  # type: ignore
     except Exception:
+        _LAST_RESOLVE_ERROR = "resolveurl missing"
         return None
 
     try:
@@ -36,7 +37,10 @@ def resolve(url: str) -> Optional[str]:
             resolver = getattr(hmf, "resolve", None)
             if callable(resolver):
                 result = resolver()
-                return str(result) if result else None
+                if result:
+                    return str(result)
+                _LAST_RESOLVE_ERROR = "unresolved"
+                return None
     except Exception as exc:
         _LAST_RESOLVE_ERROR = str(exc or "")
         pass
@@ -45,7 +49,10 @@ def resolve(url: str) -> Optional[str]:
         resolve_fn = getattr(resolveurl, "resolve", None)
         if callable(resolve_fn):
             result = resolve_fn(url)
-            return str(result) if result else None
+            if result:
+                return str(result)
+            _LAST_RESOLVE_ERROR = "unresolved"
+            return None
     except Exception as exc:
         _LAST_RESOLVE_ERROR = str(exc or "")
         return None