Better error message on `pyexec(Main, "1+1")`

[LilithHafner]

julia> pyexec(Main, "1+1")
ERROR: UndefVarError: `globals_` not defined
Stacktrace:
 [1] _pyeval_args(globals::String, locals::Nothing)
   @ PythonCall ~/.julia/packages/PythonCall/qTEA1/src/concrete/code.jl:12
 [2] pyexec(::Type{Nothing}, code::Module, globals::String, locals::Nothing)
   @ PythonCall ~/.julia/packages/PythonCall/qTEA1/src/concrete/code.jl:97
 [3] pyexec(code::Module, globals::String, locals::Nothing)
   @ PythonCall ~/.julia/packages/PythonCall/qTEA1/src/concrete/code.jl:103
 [4] pyexec(code::Module, globals::String)
   @ PythonCall ~/.julia/packages/PythonCall/qTEA1/src/concrete/code.jl:103
 [5] top-level scope
   @ REPL[8]:1

I think this should probably be a method error.

[cjdoris]

PythonCall.jl/src/concrete/code.jl

Line 9 in 16c2374

ArgumentError("globals must be a module or a Python dict")

is missing a throw(...)!