`dvc.api.params_show`: More helpful error handling

Bug Report

Description

Doesn’t show useful errors.

Reproduce

1. git clone [e-mail]:iterative/pipeline-conifguration.git
2. cd pipeline-conifguration
3. Start Python shell:

>>> import dvc.api
>>> dvc.api.params_show()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/dave/Code/dvc/dvc/api/params.py", line 267, in params_show
    return _postprocess(params)
  File "/Users/dave/Code/dvc/dvc/api/params.py", line 256, in _postprocess
    return processed[first(processed)]
KeyError: None

Expected

I’m guessing it fails because there’s no dvc.yaml or params.yaml in the project root where the shell is started, but the error message doesn’t help me understand this or how to fix it.

[daavoo]

I think an error in this case? I doubt there’s much reason to call params_show if you expect an empty dict.

I was thinking about returning an empty dict instead of raising an exception because dvc {X} show commands don’t fail when there is nothing to return.

What would happen if there was a params.yaml but no dvc.yaml found?

The contents of params.yaml will be returned.

What about the inverse?

The KeyError point will be reached.

[dberenbaum]

A warning might be reasonable also, so at least it’s not silently returning the empty dict?