Try to provide more helpful error if load_dataset is given a DataFrame

[mwaskom]

This should help reduce confusion following a relatively common beginner mistake. The current result is useless for solving the problem:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-245-abe567917c1d> in <module>
----> 1 sns.load_dataset(tips)

~/code/seaborn/seaborn/utils.py in load_dataset(name, cache, data_home, **kws)
    479                                   os.path.basename(full_path))
    480         if not os.path.exists(cache_path):
--> 481             if name not in get_dataset_names():
    482                 raise ValueError(f"'{name}' is not one of the example datasets.")
    483             urlretrieve(full_path, cache_path)

~/miniconda3/envs/seaborn-py38-latest/lib/python3.8/site-packages/pandas/core/generic.py in __nonzero__(self)
   1440     @final
   1441     def __nonzero__(self):
-> 1442         raise ValueError(
   1443             f"The truth value of a {type(self).__name__} is ambiguous. "
   1444             "Use a.empty, a.bool(), a.item(), a.any() or a.all()."

ValueError: The truth value of a DataFrame is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

Now we have

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-d18ae242c087> in <module>
----> 1 sns.load_dataset(pd.DataFrame([]))

~/code/seaborn/seaborn/utils.py in load_dataset(name, cache, data_home, **kws)
    480             "it is not necessary to use this function before plotting."
    481         )
--> 482         raise TypeError(err)
    483
    484     url = f"https://raw.githubusercontent.com/mwaskom/seaborn-data/master/{name}.csv"

TypeError: This function accepts only strings (the name of an example dataset). You passed a pandas DataFrame. If you have your own dataset, it is not necessary to use this function before plotting.

Note that this does involve a change in Exception type (but the new type is the correct one).

[codecov]

Codecov Report

Merging #2604 (19cacbc) into master (445a54a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2604   +/-   ##
=======================================
  Coverage   97.44%   97.45%           
=======================================
  Files          17       17           
  Lines        6350     6353    +3     
=======================================
+ Hits         6188     6191    +3     
  Misses        162      162           

Impacted Files	Coverage Δ
seaborn/utils.py	94.61% <100.00%> (+0.05%)	⬆️

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