The error here actually has nothing to do with dimensionless units, it has to do with scaled units.

In astropy (and in a number of unit formats) the following is valid:

```
0.01 m/s
```

However, in the FITS unit format, the scale in front is not allowed.  It juts so happens that the original error report involves a scaled, dimensionless unit (namely `%`), but this applies to any unit with a scale, dimensionless or not.

Therefore, I would say:

```
raise FitsScaleError(
    "The column {0} could not be stored in FITS format because "