Lack of metadata causes uninformative error

[JamesRunnalls]

When ozone metadata cannot be found from Nasa, for example when Sentinel3 products are processed as soon as they are published, the code returns the following error:

TypeError: object of type 'NoneType' has no len()

This is because the find function in ancillary.py returns None when it cannot find metadata and sets res to None, this causes the error above when on line 330 the len(res) is evaluated.

If the desired behavior is for Polymer to exit when there is no metadata available, maybe it would be better to raise the error in the find function and warn the user that metadata is required for processing.

Thanks