BUG: drop on axis with duplicate values doesn't raise when label is absent
#### Code Sample, a copy-pastable example if possible

```
>>> items
             item   name
store1      dress   V
store2       food   A
store2      water   C
```

CASE 1 : axis set to column and given a value which is not available in the structure. Gives ValueError.


```
>>> items.drop('abcd',axis=1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/site-packages/pandas/core/generic.py", line 2530, in drop
    obj = obj._drop_axis(labels, axis, level=level, errors=errors)
  File "/usr/local/lib/python3.6/site-packages/pandas/core/generic.py", line 2562, in _drop_axis
    new_axis = axis.drop(labels, errors=errors)
  File "/usr/local/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 3744, in drop
    labels[mask])
ValueError: labels ['abcd'] not contained in axis
```

CASE 2 : axis set to row, and given a value which is not available in structure should behave in the same way as the above case does.
Instead it just prints the structure table. Shouldn’t it be throwing ValueError ? 

```
>>> items.drop('abcd',axis=0)
             item   name
store1      dress   V
store2       food   A
store2      water   C
>>> 
```

#### Problem description

When provided invalid column name(axis=1) it throws "ValueError". Same happens with row as well(axis=0). But when there are duplicate indices in the DataFrame it doesn't.

#### Expected Output


```
>>> items.drop('d',axis=0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/site-packages/pandas/core/generic.py", line 2530, in drop
    obj = obj._drop_axis(labels, axis, level=level, errors=errors)
  File "/usr/local/lib/python3.6/site-packages/pandas/core/generic.py", line 2562, in _drop_axis
    new_axis = axis.drop(labels, errors=errors)
  File "/usr/local/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 3744, in drop
    labels[mask])
ValueError: labels ['d'] not contained in axis
```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.3.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_AU.UTF-8
LOCALE: en_AU.UTF-8

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 36.5.0
Cython: 0.27.3
numpy: 1.13.3
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>