Error handling

[xgdgsc]

Thanks for the package. Currently on IOError it only prints a warning at

slacker.jl/src/slackbot.jl

Line 164 in 2947c41

@warn "Could not send message $e"

, there seems no way I could catch an exception or get a return value indicating failure. So I cannot retry on errors. Please make the function return a boolean or throw an exception.