MIVOT: make the creation of SkyCoord instances bullet-proof

[lmichel]

While testing the mivot.features on Simbad query response, I've seen that SkyCoordBuilderoften 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.