fizzy: Handle ECHRetryAvailable explicitly

Summary:
Also enable `-Werror=switch` when compiling the Fizzy library to prevent
unhandled switch/case internally within our library.

Reviewed By: zalecodez

Differential Revision: D64440098

fbshipit-source-id: 90114e2124cd43ed17a1a08f2c5b7afbf9e57bce

fizz/fizzy_defs.bzl

@@ -8,6 +8,7 @@ load(":defs.bzl", "CXXFLAGS")
 
 FIZZY_CXXFLAGS = [
     "-fvisibility=hidden",
+    "-Werror=switch",
 ]
 
 FIZZY_PUBLIC_HEADERS = [