Warn about missing api key instead of blocking IO

[fedemengo]

With a fresh neovim setup, during plugins installation it's really annoying to have the process blocked by wakatime asking for the api key.

In my opinion it would be much better to just warn about the missing key and keep wakatime disable for the time being. Most of the plugins simply log errors/info rather than launching blocking procedures. At least it would be useful to provide an option to just log the error

[alanhamlett]

Fixed with 4faeea5.