Fix/ln unsuccessful payment (#159)

* throws error for unsuccessful payment so that wallets get error message

* bump to 0.11.2

cashu/mint/ledger.py

@@ -614,6 +614,8 @@ class Ledger:
                         ln_fee_msat=fee_msat,
                         outputs=outputs,
                     )
+            else:
+                raise Exception("Lightning payment unsuccessful.")
 
         except Exception as e:
             raise e