proto: Refactor TokenDecodeError #5182

Job	Run time
test on netbsd	5m 16s
audit	29s
features (ubuntu-latest)	1h 21m 0s
test on freebsd	4m 15s
features (macos-latest)	25m 53s
lint	1m 3s
features (windows-latest)	49m 18s
msrv	58s
test (ubuntu-latest, stable)	1m 30s
test-android (x86_64-linux-android, x86_64, 26)	2m 56s
test-aws-lc-rs	1m 38s
test (ubuntu-latest, beta)	1m 19s
test on illumos	6m 17s
test-android (i686-linux-android, x86, 26)	2m 52s
test (macos-latest, stable)	1m 6s
test on solaris	6h 0m 2s
test (windows-latest, stable)	4m 48s
test wasm32-unknown-unknown	49s
	9h 11m 29s

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