ENH: SIA should raise exception for non-existing collection

[Edit:] This has started as an astroquery issue, but looking more into it, the exception should be raised already on the pyvo level, so I transferred the issue.

```
>>> import pyvo as vo
>>> sia2 = vo.dal.sia2.SIA2Service('https://irsa.ipac.caltech.edu/SIA')
>>> sia2.search((0,0,10), collection='foobar')

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
------- ------- ------------- --------------- ------------------- ... ------- ------------ ------ -------------
```

============

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]:

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
------- ------- ------------- --------------- ------------------- ... ------- ------------ ------ -------------

```

Start with pyvo's SIA2Service.search entry point and compare it with the Irsa.query_sia example in the issue. Determine how a nonexistent collection is represented by the service, then make the completed behavior raise an exception instead of returning an empty result table; add or update coverage if an appropriate existing test location is found.

Written by the indexing model from the issue text.