Feature request: Return errors for commands after call to client.disconnect()

[TysonAndre]

Observed: After an explicit call to disconnect, commands succeed with RedisMock = require('ioredis-mock')
Expected: Commands should fail, like they do for ioredis

> RedisMock = require('.'); null
null
> r = new RedisMock(); r.disconnect(); console.log({result: await r.set('foo', 'bar')});
{ result: 'OK' }

> Redis = require('ioredis'); c = new Redis(); c.disconnect(); await c.set('foo', 'bar');
Uncaught Error: Connection is closed.

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In addition to this, it'd be useful to mock other failure modes, e.g. a read-only redis replica failing for write operations but not read operations