Better errors

[adswa]

This is what I see when attempting a file-level extraction of metadata on an untracked file:

(handbook) adina@muninn in /tmp/ds1 on git:master!
❱ datalad meta-extract -d . metalad_core newfile       
[ERROR  ] error getting status for file: /tmp/ds1/newfile:

We could probably error out a bit prettier here.

Edit: Actually, I found a variety of errors that could be proper error results. They currently all just raise, and don't perform any structured reporting:

(handbook) adina@muninn in /tmp/ds1 on git:master!
❱ datalad meta-extract -d . metalad_example_file newfile 
[ERROR  ] file not tracked: /tmp/ds1/newfile 

(handbook) adina@muninn in /tmp/ds1 on git:master!
❱ datalad meta-extract -d . metalad_example_dataset newfile 
[ERROR  ] A file-level metadata-extraction was attempted, but the specified extractor (metalad_example_dataset) is not a file-level extractor.