Option for writeMedia to log warnings and continue instead of returning at the first error

[gbogarinb]

Coming from lapwat/papeer#12
Is it possible to just log a warning instead of returning an error?
In my particular case, there seems to be a broken img link that is halting the whole process, I really don't care if a single image is not downloaded so I would still like for the process to continue.

[bmaupin]

Yes, this library needs to handle errors better. We have an open issue to improve error handling (#49) but that's for returning an error instead of using panic.

This seems like a feature request to disable the validation altogether so an error isn't thrown. I like the idea to have that as an option.

[Monirzadeh]

possible to just log a warning instead of returning an error?

@gbogarinb can you please give a code example that get you in trouble?
as i see if you use addMedia(or similar method) it can return error if it field and you can handle that yourself to log that. as i see you don't use writeMedia directly.
please review #67