Unhelpful error message when loading a single column with `read_csv` and `usecols`
#### Code
```python
>>> import pandas as pd
>>> df = pd.DataFrame({'x': [0,1], 'x1': [2,3]})
>>> df.to_csv('tmp.csv', index=False)
>>> pd.read_csv('tmp.csv', usecols='x')
   x
0  0
1  1
>>> pd.read_csv('tmp.csv', usecols=['x1'])
   x1
0   2
1   3
>>> pd.read_csv('tmp.csv', usecols='x1')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/matt/anaconda3/lib/python3.6/site-packages/pandas/io/parsers.py", line 709, in parser_f
    return _read(filepath_or_buffer, kwds)
  File "/home/matt/anaconda3/lib/python3.6/site-packages/pandas/io/parsers.py", line 449, in _read
    parser = TextFileReader(filepath_or_buffer, **kwds)
  File "/home/matt/anaconda3/lib/python3.6/site-packages/pandas/io/parsers.py", line 818, in __init__
    self._make_engine(self.engine)
  File "/home/matt/anaconda3/lib/python3.6/site-packages/pandas/io/parsers.py", line 1049, in _make_engine
    self._engine = CParserWrapper(self.f, **self.options)
  File "/home/matt/anaconda3/lib/python3.6/site-packages/pandas/io/parsers.py", line 1740, in __init__
    raise ValueError("Usecols do not match names.")
ValueError: Usecols do not match names.
```

#### Problem description
When using `usecols` to load a single column, one needs to have either a single-character column name or provide an array-like object.  In the example above, `pd.read_csv('tmp.csv', usecols='x')` and `pd.read_csv('tmp.csv', usecols=['x1'])` work as expected; however, things break down for `pd.read_csv('tmp.csv', usecols='x1')`.  The corresponding error message `ValueError: Usecols do not match names.` is not very helpful either. 

#### Expected Output
It would be nice if there were some type checking done on `usecols` so that things don't break in the example above.   At the least, the error message should be a bit more helpful; e.g., `ValueError: Usecols should be array-like.`


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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Linux
OS-release: 4.13.0-37-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 39.0.1
Cython: None
numpy: 1.14.2
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.7.1
patsy: None
dateutil: 2.7.1
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.9999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
