Table.write to FITS should maybe not crash if a unit is not valid in FITS

At the moment, Table.write simply crashes if a unit is not valid in the FITS format:

```
ValueError: 'ct' did not parse as fits unit: At col 0, Unit u'c' not supported by the FITS standard. Did you mean C?
```

Maybe this should simply be a warning?
Looks like this went along for the ride here as well :)