PR feedbacks

- Move exceptions to exceptions.py
- Raise `UpgradeFailure` in upgrader
- Refine the try/catch for upgraders in swarm

libp2p/protocol_muxer/exceptions.py

@@ -0,0 +1,9 @@
+from libp2p.exceptions import BaseLibp2pError
+
+
+class MultiselectError(BaseLibp2pError):
+    """Raised when an error occurs in multiselect process"""
+
+
+class MultiselectClientError(BaseLibp2pError):
+    """Raised when an error occurs in protocol selection process"""