clearer error message for default MacOS matplotlib

I've imported the PyPlot module like so:
`using PyCall`
`pygui(:tk)`
`using PyPlot`

I then try to use the plot function:
`x=1:10; y=sin(x);`
`plot(x,y)`

But I get this error:
`pltm not defined`
`while loading In[14], in expression starting on line 1`

`in abstract_eval_global at ./inference.jl:1027`
`in abstract_eval_symbol at ./inference.jl:1066`
`in abstract_eval at ./inference.jl:919`
`in abstract_eval_arg at ./inference.jl:866`
`in abstract_eval_call at ./inference.jl:878`
`in abstract_eval at ./inference.jl:935`
`in abstract_eval_arg at ./inference.jl:866`
`in abstract_eval_call at ./inference.jl:878`
`in abstract_eval at ./inference.jl:935`
`in abstract_interpret at ./inference.jl:1107`
`in typeinf at ./inference.jl:1409`
`in typeinf at /Users/tre11/git/julia/usr/lib/julia/sys.dylib`
`in typeinf_ext at ./inference.jl:1216`

This is in the latest stable version of Julia 0.3.4-pre+8, and latest stable versions of python, ipython, matpplotlib, etc.

Start by reproducing the reported Julia 0.3.4 sequence with PyCall, pygui(:tk), PyPlot, and plot(x,y) on macOS. Trace where the `pltm not defined` error is produced; done means the default macOS matplotlib setup reports a clear, actionable error.

Scritto dal modello di indicizzazione a partire dal testo della issue.