provide helpful message on output duplications

[skshetry]

Bug Report

Repro

$ cat dvc.yaml
stages:
  plot-metrics:
    cmd: cp metrics-temp.csv metrics.csv
    deps:
    - metrics-temp.csv
    plots:
    - metrics.csv
  generate-foo:
    cmd: echo 'foo0\nfoo1' > foo && cat foo
    outs:
    - foo
    - metrics.csv  # notice the output duplication
$ python -c '
import dvc.api

with dvc.api.open("metrics.csv") as f:
  print(f.read())'

Traceback (most recent call last):
  File "/home/saugat/repos/iterative/dvc/dvc/repo/__init__.py", line 585, in open_by_relpath
    remote=remote,
  File "/home/saugat/repos/iterative/dvc/dvc/repo/tree.py", line 257, in open
    path, mode=mode, encoding=encoding, **kwargs
  File "/home/saugat/repos/iterative/dvc/dvc/repo/tree.py", line 67, in open
    raise IsADirectoryError
IsADirectoryError

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<string>", line 4, in <module>
  File "/home/saugat/.pyenv/versions/3.6.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "/home/saugat/repos/iterative/dvc/dvc/api.py", line 74, in _open
    path, remote=remote, mode=mode, encoding=encoding
  File "/home/saugat/.pyenv/versions/3.6.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "/home/saugat/repos/iterative/dvc/dvc/repo/__init__.py", line 591, in open_by_relpath
    raise DvcIsADirectoryError(f"'{path}' is a directory") from exc
dvc.exceptions.IsADirectoryError: '/home/saugat/dvc_try/test_multistage/metrics.csv' is a directory

Similar things happens on plots diff (message is cryptic):

$ dvc plots diff -t metrics.csv HEAD~
ERROR: unexpected error

Having any troubles? Hit us up at https://dvc.org/support, we are always happy to help!
$ dvc plots diff -t metrics.csv HEAD~ -v
2020-06-10 18:28:50,571 ERROR: unexpected error
------------------------------------------------------------
Traceback (most recent call last):
  File "/home/saugat/repos/iterative/dvc/dvc/main.py", line 53, in main
    ret = cmd.run()
  File "/home/saugat/repos/iterative/dvc/dvc/command/plots.py", line 53, in run
    plots = self._func(targets=self.args.targets, props=self._props())
  File "/home/saugat/repos/iterative/dvc/dvc/command/plots.py", line 88, in _func
    return self.repo.plots.diff(*args, revs=self.args.revisions, **kwargs)
  File "/home/saugat/repos/iterative/dvc/dvc/repo/plots/__init__.py", line 82, in diff
    return diff(self.repo, *args, **kwargs)
  File "/home/saugat/repos/iterative/dvc/dvc/repo/plots/diff.py", line 12, in diff
    *args, revs=_revisions(revs, repo.scm.is_dirty()), **kwargs
  File "/home/saugat/repos/iterative/dvc/dvc/repo/plots/__init__.py", line 65, in show
    data = self.collect(targets, revs)
  File "/home/saugat/repos/iterative/dvc/dvc/repo/plots/__init__.py", line 41, in collect
    with tree.open(datafile) as fd:
  File "/home/saugat/repos/iterative/dvc/dvc/repo/tree.py", line 257, in open
    path, mode=mode, encoding=encoding, **kwargs
  File "/home/saugat/repos/iterative/dvc/dvc/repo/tree.py", line 67, in open
    raise IsADirectoryError
IsADirectoryError
------------------------------------------------------------

Please provide information about your setup

Output of dvc version:

$ dvc version
dvc version
DVC version: 1.0.0a10+91a719
Python version: 3.6.6
Platform: Linux-5.5.2-arch1-1-x86_64-with-arch
Binary: False
Package: None
Supported remotes: azure, gdrive, gs, hdfs, http, https, s3, ssh, oss
Cache: reflink - not supported, hardlink - supported, symlink - supported
Filesystem type (cache directory): ('ext4', '/dev/sda9')
Repo: dvc, git
Filesystem type (workspace): ('ext4', '/dev/sda9')

Additional Information (if any):

Expected Behavior

Complain that output is duplicated. The error message is not helpful at all.

We do provide helpful message on run/repro/add/imp, but not on these checkouts.
Also, we should not throw out error on dvc checkout foo as it will affect workflow.

[skshetry]

Also, with our effort to enable user for easier writability, this might be a common occurence, and better output message would be very helpful here.