BUG: Adding `label` argument to `plot()` removes error bars
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---
#### Code Sample, a copy-pastable example

```python
import pandas as pd
import matplotlib.pyplot as plt


cols = ['x']
val = pd.DataFrame([1,2], columns=cols)
err = pd.DataFrame([0.5,0.5], columns=cols)

# Has error bars
val.plot(y=cols, yerr=err)

# Does not have error bars, but should
val.plot(y=cols, yerr=err, label=["label"])

plt.show()
```

#### Problem description

When adding both `yerr` and `label` as parameters to the `plot()` function call, no error bars are plotted.

#### Expected Output

Both plots should look identical apart from differing labels, but the second one does not have error bars.

#### Output of ``pd.show_versions()``

<details>

commit           : bd91f4577c5b54a72a4e092a7d7db43fec1b46bb
python           : 3.8.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.5.13-arch2-1
Version          : #1 SMP PREEMPT Mon, 30 Mar 2020 20:42:41 +0000
machine          : x86_64
processor        :
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 1.1.0.dev0+1159.gbd91f4577c
numpy            : 1.18.2
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.1.3
Cython           : 0.29.16
pytest           : 5.4.1
hypothesis       : None
sphinx           : 3.0.0
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : 2.11.1
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.2.1
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.15
tables           : None
tabulate         : 0.8.7
xarray           : None
xlrd             : None
xlwt             : None
numba            : None
</details>
