BUG: XMatch two Vizier catalogs then no col refs should be added

[bsipocz]

When matching two catalogs from Vizier no reference columns should be specified, currently we're wrongly raise an exception:

In [17]: from astropy import units as u
    ...: from astroquery.xmatch import XMatch
    ...: table = XMatch.query_async(cat1='vizier:J/A+A/331/81/table2', cat2='vizier:IV/34/epic', max_distance=5 * u.arcsec)
    ...: 
    ...: 
    ...: 
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-17-6263e825bbc7> in <module>()
      1 from astropy import units as u
      2 from astroquery.xmatch import XMatch
----> 3 table = XMatch.query_async(cat1='vizier:J/A+A/331/81/table2', cat2='vizier:IV/34/epic', max_distance=5 * u.arcsec)
      4 

~/munka/devel/astroquery/astroquery/xmatch/core.py in query_async(self, cat1, cat2, max_distance, colRA1, colDec1, colRA2, colDec2, cache, get_query_payload)
     83         kwargs = {}
     84 
---> 85         self._prepare_sending_table(1, payload, kwargs, cat1, colRA1, colDec1)
     86         self._prepare_sending_table(2, payload, kwargs, cat2, colRA2, colDec2)
     87 

~/munka/devel/astroquery/astroquery/xmatch/core.py in _prepare_sending_table(self, i, payload, kwargs, cat, colRA, colDec)
    115         if not self.is_table_available(cat):
    116             if ((colRA is None) or (colDec is None)):
--> 117                 raise ValueError('Specify the name of the RA/Dec columns in' +
    118                                  ' the input table.')
    119             # if `cat1` is not a VizieR table,

ValueError: Specify the name of the RA/Dec columns in the input table.

[ManonMarchand]

This is not a bug, J/A+A/331/81/table2 is not available on the cross match server because this VizieR table has no coordinates and cannot be crossmatched.

Since the error message was confusing, the linked PR tries to make the two possible reasons for failure clear.