feat: consolidate mTLS channel errors (#480)

feat: consolidate mTLS channel errors
diff --git a/google/auth/exceptions.py b/google/auth/exceptions.py
index 4f66dc2..9501386 100644
--- a/google/auth/exceptions.py
+++ b/google/auth/exceptions.py
@@ -34,3 +34,7 @@
 
 class DefaultCredentialsError(GoogleAuthError):
     """Used to indicate that acquiring default credentials failed."""
+
+
+class MutualTLSChannelError(GoogleAuthError):
+    """Used to indicate that mutual TLS channel creation is failed."""