invoices: always validate that LNInvoice.invoice can be decoded

related: #6559

The LNInvoice.from_json() method previously did not validate, which is
used by e.g. wallet.import_invoices.

electrum/invoices.py

@@ -149,6 +149,10 @@ class LNInvoice(Invoice):
 
     __lnaddr = None
 
+    @invoice.validator
+    def check(self, attribute, value):
+        lndecode(value)  # this checks the str can be decoded
+
     @property
     def _lnaddr(self) -> LnAddr:
         if self.__lnaddr is None:

electrum/lnutil.py

@@ -1036,7 +1036,8 @@ def validate_features(features: int) -> None:
         if (1 << fbit) & LN_FEATURES_IMPLEMENTED == 0 and fbit % 2 == 0:
             raise UnknownEvenFeatureBits(fbit)
     if not features.validate_transitive_dependecies():
-        raise IncompatibleOrInsaneFeatures("not all transitive dependencies are set")
+        raise IncompatibleOrInsaneFeatures(f"not all transitive dependencies are set. "
+                                           f"features={features}")
 
 
 def derive_payment_secret_from_payment_preimage(payment_preimage: bytes) -> bytes: