Improve connections error reporting

[emanuele-f]

When an error occurs for a connection (e.g. abnormal close), currently PCAPdroid only reports a generic "error". This should be characterized in more detail, to give the user some more info about the error.

For example, using the security.cloudflare-dns.com safe DNS, results in frequent errors. Internally, zdtun logs a TCP send error[32]: Broken pipe. Investigating this in root mode reveals that the server closes the connection with a TCP RST after the client sends the TCP FIN.