better undefined reference handling in show()

[nolta]

I'm not sure if this is a bug, but currently:

julia> x = cell(2,1)
2x1 Any Array:
access to undefined reference

After this patch:

julia> x = cell(2,2)
2x2 Any Array:
 undef  undef
 undef  undef

julia> x[1,1] = 1.
2x2 Any Array:
  1.0   undef
 undef  undef

[ViralBShah]

I think it is better to show undef rather than a strange error message that sounds scary. @JeffBezanson and @StefanKarpinski should also take a look. Should we have undef as a type?

[StefanKarpinski]

Awesome. @ViralBShah: it makes no sense to have an Undef type because the whole point of our undefined value behavior is that no usable value can be undefined. If we had an Undef type, we'd be in the same sort of nasty situation as Java, checking for null all over the place.