Add concrete Error implementation for mixnet #250

Job	Run time
Rust lints (libp2p)	3m 32s
Check (libp2p)	2m 48s
Rust lints (waku)	4m 40s
Check (waku)	4m 58s
Test Suite (libp2p, ubuntu-latest)	8m 32s
Test Suite (libp2p, windows-latest)	20m 55s
Test Suite (libp2p, macos-latest)	10m 50s
Test Suite (waku, ubuntu-latest)	10m 34s
Test Suite (waku, windows-latest)	21m 43s
Test Suite (waku, macos-latest)	15m 26s
	1h 43m 58s

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