show error message raised during sign_transaction. see issue #1422

lib/wallet.py

@@ -1023,7 +1023,12 @@ class Abstract_Wallet(object):
         # Sign
         if keypairs:
             tx.sign(keypairs)
+        # Run hook, and raise if error
+        tx.error = None
         run_hook('sign_transaction', tx, password)
+        if tx.error:
+            raise BaseException(tx.error)
+
 
     def sendtx(self, tx):
         # synchronous