reshape: helpful error message, more tests

[timholy]

I was intending to add a NEWS item (since added by Jeff), but discovered a few more loose ends. This adds an error message:

 julia> a = reshape(1:12, 4, 3)
4×3 Base.ReshapedArray{Int64,2,UnitRange{Int64},Tuple{}}:
 1  5   9
 2  6  10
 3  7  11
 4  8  12

julia> a[3,2] = -1
ERROR: indexed assignment fails for a reshaped range; consider calling collect
 in setindex!(::Base.ReshapedArray{Int64,2,UnitRange{Int64},Tuple{}}, ::Int64, ::Int64, ::Int64) at ./reshapedarray.jl:138
 in eval(::Module, ::Any) at ./boot.jl:236

This also tests more explicitly for the sharing behavior than before.

[tkelman]

if this is supposed to always fail, needs an error("unexpected") or something so it doesn't silently pass

[timholy]

Good catch, thanks.

[timholy]

f5810f2