client: Properly report getaddrinfo() failing

getaddrinfo() does not set errno, so return -EADDRNOTAVAIL instead if it
fails.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
client: Add IPv6 support

Set the address family in the hints to AF_UNSPEC, so both IPv4 and IPv6
addresses can be resolved.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
src/riemann-client: Better error handling

When the connection or the message sending fails, notify the user.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>