Add default to function generating name of Transport Error Code

Function converting TransportErrorCode from a number to a string
did not have a default statement.  If the error code was not one
of the predefined ones, this would cause a test failure. This CL
adds a proper default: clause.

gfe-relnote: N/A test/V99/IETF-QUIC code only.
PiperOrigin-RevId: 264136976
Change-Id: I7aa7b6d3615d4b12e7de03ca913fb23a989b0734

quic/core/quic_types.cc

diff --git a/quic/core/quic_types.cc b/quic/core/quic_types.cc
index 2fe951b..a8b8db9 100644
--- a/quic/core/quic_types.cc
+++ b/quic/core/quic_types.cc
@@ -99,7 +99,8 @@
     RETURN_STRING_LITERAL(VERSION_NEGOTIATION_ERROR);
     RETURN_STRING_LITERAL(PROTOCOL_VIOLATION);
     RETURN_STRING_LITERAL(INVALID_MIGRATION);
-    // No default -- compiler will catch any adds/changes then.
+    default:
+      return QuicStrCat("Unknown Transport Error Code Value: ", c);
   }
 }