MIVOT: make the creation of SkyCoord instances bullet-proof

While testing the `mivot.features` on Simbad query response, I've seen that `SkyCoordBuilder`often fails due to the lack of input data, or even worse due to their inconsistency.
Simbad is a very good test bed because the data aggregated in query response are sparse.

- The missing/wrong parameters must be checked individually and possibly flagged as ignored
- Errors risen by Astropy when the instantiation fails, must be properly caught.
- A specific test module must be setup for this purpose.

Start by locating SkyCoordBuilder and the MIVOT handling used for Simbad query responses. Add a dedicated test module covering missing or inconsistent parameters, ignored values, and Astropy instantiation failures; done means these cases are handled without uncaught errors.

Written by the indexing model from the issue text.