Give a better error when the picture regex fails

[krowvin]

A user forgot to place f in their PICTURE string for
%5.0f and instead entered %5.0

Suggest catching this error and improving the output

File "/wm/swl/m4cwpa50/.local/lib/python3.9/site-packages/repgen/data/value.py", line 824, in format
    picture = re.search(r"%([0-9.,+-]*[bcdoxXneEfFgG%])", self.picture).group(1)
AttributeError: 'NoneType' object has no attribute 'group'