Better parse errors

[mauro3]

These should both error and error better:

julia> @traitfn f(x::X, y:Y) where {X, Y; T1{X}, T2{X}} = 1
ERROR: LoadError: Cannot parse argument: y:Y
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] strip_kw at /home/mauro/julia/dot-julia-dev/SimpleTraits/src/SimpleTraits.jl:429 [inlined]
 [3] strip_kw(::Array{Any,1}) at /home/mauro/julia/dot-julia-dev/SimpleTraits/src/SimpleTraits.jl:420
 [4] traitfn(::Expr, ::Module) at /home/mauro/julia/dot-julia-dev/SimpleTraits/src/SimpleTraits.jl:331
 [5] @traitfn(::LineNumberNode, ::Module, ::Any) at /home/mauro/julia/dot-julia-dev/SimpleTraits/src/SimpleTraits.jl:388
in expression starting at REPL[15]:1

julia> @traitfn f(x::X, y::Y) where {X, Y; T1{X}, T2{X}} = 1
f (generic function with 2 methods)