proto: Refactor TokenDecodeError #5198

Job	Run time
test on illumos	6m 47s
test on solaris	4m 55s
lint	1m 3s
msrv	1m 5s
audit	22s
test (ubuntu-latest, stable)	1m 36s
test-android (x86_64-linux-android, x86_64, 26)	2m 48s
test (ubuntu-latest, beta)	1m 14s
test-aws-lc-rs	1m 42s
test-android (i686-linux-android, x86, 26)	2m 45s
test on freebsd	3m 56s
test on netbsd	4m 52s
test wasm32-unknown-unknown	57s
test (macos-latest, stable)	1m 12s
test (windows-latest, stable)	3m 50s
features (ubuntu-latest)	1h 22m 27s
features (macos-latest)	25m 35s
features (windows-latest)	50m 43s
	3h 17m 49s

Provide feedback