if a private channel is deleted in slack, user was not dropped from irslackd channel

[jcolson]

Description
A clear and concise description of what the bug is.

Steps to reproduce

1. ...create channel, join it, with any client
2. ...use slack client, delete channel
3. ...irslackd user still shows being in channel and /part doesn't work

Expected behavior
user should be removed from channel and/or be able to /part a nonexistant channel

Log output

 1 Feb 27 16:43:48 vpc irslackd[1039]: slack_in user_typing { type: 'user_typing', channel: 'D43KMNF16', user: 'U0AAQ3HQ9' }
 2 Feb 27 16:43:48 vpc irslackd[1039]: slack_out conversations.leave { channel: 'CGJ9UEYUA' }
 3 Feb 27 16:43:48 vpc irslackd[1039]: irc_in PART #sev_sfr Buh-bye!
 4 Feb 27 16:43:48 vpc irslackd[1039]: Trace: irslackd_err Failed onIrcPart: Error: {
 5 Feb 27 16:43:48 vpc irslackd[1039]: "method": "conversations.leave",
 6 Feb 27 16:43:48 vpc irslackd[1039]: "options": {
 7 Feb 27 16:43:48 vpc irslackd[1039]: "channel": "CGJ9UEYUA"
 8 Feb 27 16:43:48 vpc irslackd[1039]: },
 9 Feb 27 16:43:48 vpc irslackd[1039]: "error": {
10 Feb 27 16:43:48 vpc irslackd[1039]: "code": "slackclient_platform_error",
11 Feb 27 16:43:48 vpc irslackd[1039]: "data": {
12 Feb 27 16:43:48 vpc irslackd[1039]: "ok": false,
13 Feb 27 16:43:48 vpc irslackd[1039]: "error": "channel_not_found",
14 Feb 27 16:43:48 vpc irslackd[1039]: "scopes": [
15 Feb 27 16:43:48 vpc irslackd[1039]: "read",
16 Feb 27 16:43:48 vpc irslackd[1039]: "client",
17 Feb 27 16:43:48 vpc irslackd[1039]: "admin",
18 Feb 27 16:43:48 vpc irslackd[1039]: "identify",
19 Feb 27 16:43:48 vpc irslackd[1039]: "post",
20 Feb 27 16:43:48 vpc irslackd[1039]: "apps"
21 Feb 27 16:43:48 vpc irslackd[1039]: ],
22 Feb 27 16:43:48 vpc irslackd[1039]: "acceptedScopes": [
23 Feb 27 16:43:48 vpc irslackd[1039]: "channels:write",
24 Feb 27 16:43:48 vpc irslackd[1039]: "groups:write",
25 Feb 27 16:43:48 vpc irslackd[1039]: "mpim:write",
26 Feb 27 16:43:48 vpc irslackd[1039]: "im:write",
27 Feb 27 16:43:48 vpc irslackd[1039]: "post"
28 Feb 27 16:43:48 vpc irslackd[1039]: ]
29 Feb 27 16:43:48 vpc irslackd[1039]: }
30 Feb 27 16:43:48 vpc irslackd[1039]: }
31 Feb 27 16:43:48 vpc irslackd[1039]: }
32 Feb 27 16:43:48 vpc irslackd[1039]: at WebClient.apiCallOrThrow (/home/ec2-user/src/irslackd/lib/irslackd.js:1268:13)
33 Feb 27 16:43:48 vpc irslackd[1039]: at <anonymous>
34 Feb 27 16:43:48 vpc irslackd[1039]: at process._tickCallback (internal/process/next_tick.js:189:7)
35 Feb 27 16:43:48 vpc irslackd[1039]: at Irslackd.logError (/home/ec2-user/src/irslackd/lib/irslackd.js:1059:13)
36 Feb 27 16:43:48 vpc irslackd[1039]: at res.catch (/home/ec2-user/src/irslackd/lib/irslackd.js:1231:18)
37 Feb 27 16:43:48 vpc irslackd[1039]: at <anonymous>
38 Feb 27 16:43:48 vpc irslackd[1039]: at process._tickCallback (internal/process/next_tick.js:189:7)
39 Feb 27 16:43:49 vpc irslackd[1039]: slack_in pong { type: 'pong', reply_to: 1564 }
40 Feb 27 16:43:49 vpc irslackd[1039]: slack_in pong { type: 'pong', reply_to: 25602 }
41 Feb 27 16:43:50 vpc irslackd[1039]: irc_in WHO #randomtest

Client setup

• IRC client and version:quassel 13.1
• IRC bouncer and version (if any):quassel 13.1
• OS:amazon linux2

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Tracking here #75