make the process error with a proper exit code

[staabm]

Inspired by https://medium.com/@dtinth/making-unhandled-promise-rejections-crash-the-node-js-process-ffc27cfcc9dd

should close https://github.com/agneym/generate-og-image/issues/34

Be warned: I am not all all a nodejs expert and this change was not tested

[staabm]

The built contains the same errors reported in https://github.com/agneym/generate-og-image/issues/33 which IMO are unrelated