fix reward channel joining

[ayrat555]

fixes

2019-06-13T21:04:22.331 application=phoenix [error] an exception was raised:
    ** (MatchError) no match of right hand side value: {:error, :not_found}
        (block_scout_web) lib/block_scout_web/channels/reward_channel.ex:15: BlockScoutWeb.RewardChannel.join/3
        (phoenix) lib/phoenix/channel/server.ex:245: Phoenix.Channel.Server.init/1
        (stdlib) gen_server.erl:374: :gen_server.init_it/2
        (stdlib) gen_server.erl:342: :gen_server.init_it/6
        (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3

Changelog

• fix reward channel joining

[coveralls]

Pull Request Test Coverage Report for Build 4f36c651-d40e-4456-b042-3ce57be54d2e

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 80.762%

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Totals
Change from base Build be7d4c0a-8fbe-49ce-ae62-2e3ddf25e349:	0.0%
Covered Lines:	4899
Relevant Lines:	6066

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💛 - Coveralls