Add isErrored() function

[srd424]

Add isErrored() function so we can tell if the context / connection is in the "errored" state, as may occur if the initial TCP connection fails.

(This commit has been sitting in my fork for 18 months, not sure why I never turned it into a PR before - doing it now in preparation for tidying up my repo list as I move slowly to codeberg. I can't actually remember why I needed this, but I'm pretty sure I did - if you'd like me to go digging for the calling code I used it in to demonstrate the requirement, let me know!)