Error message for a partially implemented plan

[dlfivefifty]

This avoids the following stack overflow:

[1] circcopy!(::Array{Float64,2}, ::Array{Float64,2}) at ./multidimensional.jl:1071
 [2] copy1 at /Users/solver/.julia/packages/AbstractFFTs/JAxy0/src/definitions.jl:38 [inlined]
 [3] *(::SphericalHarmonics.SphericalHarmonicTransform{Float64}, ::Array{Float64,2}) at /Users/solver/.julia/packages/AbstractFFTs/JAxy0/src/definitions.jl:208 (repeats 51959 times)

Haven't had time to add a test but can do it once the implementation is agreed (that is, the error type/message).