More error tolerant attempt to save /fetch remote image

[PhilGale92]

Heya! one more quick potential change from me to gather thoughts.

Im using this tool to use SSG by conecting to contentful and downloading all the remote images. Now sometimes contentful fails to provide the image. Would it be possible to add a mode to downloadImagesInBatches in which the Unable to save ${formattedFilename} (empty file). error (and maybe other errors). Resolve() out rather than rejection.

Just as otherwise i'd have to query every image first / it seems a shame for the whole process to die out because of one image-download failure.

Hope the above makes sense, again if you okay it im happy to give it a go to make the change. Thanks,