read_table should check for engine name validity

[wavexx]

We should really check for valid engine names for the engine argument to pd.read_table. An invalid name results in a dubious error to the user:

pd.read_table('', engine='test')
>>> Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/pandas/io/parsers.py", line 646, in parser_f
    return _read(filepath_or_buffer, kwds)
  File "/usr/lib/python3/dist-packages/pandas/io/parsers.py", line 389, in _read
    parser = TextFileReader(filepath_or_buffer, **kwds)
  File "/usr/lib/python3/dist-packages/pandas/io/parsers.py", line 730, in __init__
    self._make_engine(self.engine)
  File "/usr/lib/python3/dist-packages/pandas/io/parsers.py", line 929, in _make_engine
    self._engine = klass(self.f, **self.options)
UnboundLocalError: local variable 'klass' referenced before assignment

[chris-b1]

Thanks for the report, this was closed by #16511 and is fixed in 0.20.2