refactor: use thiserror instead of failure

[kurnevsky]

Fixes #424

[coveralls]

Pull Request Test Coverage Report for Build 1274747141

• 211 of 311 (67.85%) changed or added relevant lines in 20 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.3%) to 87.197%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tox_core/src/dht/lan_discovery.rs	0	1	0.0%
tox_core/src/dht/codec.rs	4	6	66.67%
tox_core/src/dht/server_ext.rs	5	7	71.43%
tox_core/src/relay/server/server_ext.rs	0	4	0.0%
tox_core/src/friend_connection/mod.rs	7	12	58.33%
tox_core/src/relay/client/client.rs	38	45	84.44%
tox_core/src/onion/client/mod.rs	20	30	66.67%
tox_core/src/relay/client/connections.rs	11	22	50.0%
tox_core/src/udp.rs	0	12	0.0%
tox_core/src/net_crypto/mod.rs	42	64	65.63%

Files with Coverage Reduction	New Missed Lines	%
tox_core/src/net_crypto/mod.rs	1	90.3%

Totals
Change from base Build 1273276378:	0.3%
Covered Lines:	14336
Relevant Lines:	16441

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💛 - Coveralls

[kpp]

Continue?

[kurnevsky]

I inverted the condition - we ignore only decoding errors, not IO.