Handle premature websocket close

[mnemitz]

• Add logic ensuring if socket closes before the ConversationStarted message is received, we give up waiting for it
• Add test
• Add cancellable functionality to timeouts, so the test scripts don't hang on an orphaned promise
• Add error type strings to the SpeechmaticsFlowError class, so we can handle different errors differently
• Add the "ws" dependency to the flow-client package (this was missing before, breaking the Node polyfill)