plots: improve non-out error message

[jorgeorpinel]

Per #3994 (comment)

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

• ❌ I will check DeepSource, CodeClimate, and other sanity checks below. (We consider them recommendatory and don't expect everything to be addressed. Please fix things that actually improve code or fix bugs.)

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[jorgeorpinel]

❗ I have followed the Contributing to DVC checklist.

The Git hook failed on my machine with:

pylint...................................................................Failed
- hook id: pylint
- exit code: 1

Traceback (most recent call last):
  File "c:\users\poj12\appdata\local\programs\python\python38\lib\runpy.py", line 193, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "c:\users\poj12\appdata\local\programs\python\python38\lib\runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "C:\Users\poj12\dvc\.venv\Scripts\pylint.EXE\__main__.py", line 7, in <module>
  File "c:\users\poj12\dvc\.venv\lib\site-packages\pylint\__init__.py", line 22, in run_pylint
    PylintRun(sys.argv[1:])
  File "c:\users\poj12\dvc\.venv\lib\site-packages\pylint\lint\run.py", line 303, in __init__
    linter.load_plugin_modules(plugins)
  File "c:\users\poj12\dvc\.venv\lib\site-packages\pylint\lint\pylinter.py", line 499, in load_plugin_modules
    module.register(self)
  File "c:\users\poj12\dvc\.venv\lib\site-packages\pylint_pytest\__init__.py", line 24, in register
    checker = importlib.import_module(module, package=os.path.basename(dirname))
  File "c:\users\poj12\appdata\local\programs\python\python38\lib\importlib\__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 973, in _find_and_load_unlocked
ModuleNotFoundError: No module named '\\checkers\\class_attr_loader'

So I had to skip it to commit. Let's see if the tests pass here...

[codecov]

Codecov Report

Merging #4115 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4115   +/-   ##
=======================================
  Coverage   92.45%   92.46%           
=======================================
  Files         162      162           
  Lines       11277    11277           
=======================================
+ Hits        10426    10427    +1     
+ Misses        851      850    -1     

Impacted Files	Coverage Δ
dvc/repo/plots/__init__.py	92.42% <ø> (ø)
dvc/output/base.py	95.69% <0.00%> (+0.35%)	⬆️

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[jorgeorpinel]

Applied your suggestion @efiop thanks! The tests had only failed on Windows, not sure why, maybe a glitch. Let's see this time.