Improved error handling

[pulsastrix]

Summary

libcoap-rs currently has rather limited error-handling capabilities. This should be improved.

Description

While libcoap-rs has some error handling support, it falls short in some regards:

• No library-internal logging
  • The log crate would allow library users to use their own logging framework while still allowing us to provide log output
• Some errors are silently ignored
  • While message parsing errors are responded to with an RST message, the library user is not informed of such malformed requests. It might be useful if users could provide callbacks/event handlers for such events
• We might want to re-evaluate if there are some places where we panic even though we should return a Result
• libcoap is rather limited in its error output: Most functions simply return a value on success and NULL on failure, making it rather hard to pinpoint the exact reason for this error.
  • Fixing this would probably require some large changes in libcoap as we would have to implement error codes throughout the entire library

Implementation steps

• Add some library-internal logging
  • Integrate the log crate and add logging messages where reasonable
  • Redirect libcoap's own logging to the log crate (see coap_set_log_handler())
• Add event handler methods for error handling
• Evaluate all panics
• (Optional: Add error codes and error code retrieval functions to libcoap)

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