meta-add allows adding file-level metadata with non-existent path

[adswa]

I played around with metalad commands in the context of #291. Something that surprised me was the following:

If I extract metadata into a file, and then alter the path property in the metadata record to a non-existent file, meta-add will happily let me add this metadata record to the dataset.

Do you think there should be a check that the paths metadata belongs to are actually existing?