Add informative error for missing PyCall library
Add informative error for missing PyCall library

Yields the following error

Julia does not have package PyCall.
Install PyCall by running the following line:
    julia -e 'Pkg.add("PyCall"); Pkg.update()'

JuliaPy/pyjulia,JuliaLang/pyjulia,JuliaPy/pyjulia,PallHaraldsson/pyjulia