Add more guidance on error handling in the guide/docs

[Azmisov]

The guide says that errors should be handled, but doesn't really give any guidance on what to do for certain errors.

For example, when sending a multipart message and I get an error what should I do? Is the socket/context invalid now and need to be restarted? Should I resend the full multipart message? Should I send just the part for which the error occurred?

The example "error handling" in the guide just uses assertions, so the program just gets killed on error. Would be great if there were more details on how to do graceful error recovery? The guide mentions there are only two "nonfatal" error scenarios (nonblocking receive, context destroy); I'm assuming the wording "nonfatal" means the errors returned in these cases are informational rather than true errors, not that all other error scenarios necessitate your program to die.

relevant portion of guide