BUG: SIA should raise exception for non-existing collection

[bsipocz]

Currently it's possible to query a collection that doesn't exist on the server and receive an empty result table. I would think a better UX would be to receive an exception in these cases.

E.g. currently this is what I get:

In [49]: Irsa.query_sia(pos=(coord, 10), collection='foobar')
Out[49]: 
<DALResultsTable length=0>
  s_ra   s_dec  facility_name instrument_name dataproduct_subtype ... pol_xel cloud_access o_ucd  upload_row_id
  deg     deg                                                     ...                                          
float64 float64     object         object            object       ...  int64     object    object     int64    
------- ------- ------------- --------------- ------------------- ... ------- ------------ ------ -------------

Note that we have looked into addressing this upstream in pyvo, but unfortunately there is no good solutions for this.
So this issue is for specifically for IRSA where we have the possible collection values, thus we cache it for the instance.

x-ref: astropy/pyvo#651