Add a warning when the shape palette is exceeded

[jiho]

This warning was present in ggplot before and is necessary to understand why
shapes beyond the 6th are suppresses. But this being outside of the scope of
ggplot, I did not know how to phrase the warning exactly.

For information, the warning in ggplot was:

scale_shape_discrete can deal with
a maximum of 6 discrete values, but you have 11.  See ?scale_manual for a
possible alternative

[hadley]

Thanks - I made a few tweaks.