fix: initialize error variables before retry loops

Ensure error variables are initialized before entering retry loops in
ClusterLib and TransactionGroup classes. This prevents potential issues
with uninitialized variables and improves code clarity.

Author: mkoura

cardano_clusterlib/clusterlib_klass.py

@@ -267,6 +267,7 @@ def cli(
         # Network.Socket.connect: <socket: X>: resource exhausted (Resource temporarily unavailable)
         # or
         # MuxError (MuxIOException writev: resource vanished (Broken pipe)) "(sendAll errored)"
+        err_msg = ""
         for __ in range(3):
             retcode = None
             with subprocess.Popen(

cardano_clusterlib/transaction_group.py

@@ -1505,6 +1505,7 @@ def submit_tx(
             else wait_blocks
         )
         txid = ""
+        err = None
         for r in range(20):
             err = None