Replace ranch errors with prettier errors in channels

Closes #154 

From this:
![ranch-errors](https://www.evernote.com/shard/s280/sh/d983f323-796c-4b80-b777-6f0727169df7/4a713d595a2166f82a383ab1f6c25410/deep/0/Screenshot%2010/9/14,%201:38%20AM.jpg)

To this:
![better-errors](https://www.evernote.com/shard/s280/sh/15dddfae-5b1a-40fb-a7de-402ac17e109a/cca19b9fa4095371fe4f79e12c93f8ca/deep/0/Screenshot%2010/9/14,%201:37%20AM.jpg)

How do I add tests for this? I looked at `handler_test.exs` and it doesn't have any tests for `websocket_handle/3`. 
`assert error_banner =~ "(RuntimeError) Foo"`