Need better error validation for improper inputs

[ViralBShah]

I provided an improper data structure for labels to and got the following error. I was providing a vector of tuples instead of what it expects. I think validating the types of the inputs and providing a better error message would be helpful.

julia> bst = xgboost(s, 20, label = labels, eta = 1, max_depth = 2)
ERROR: MethodError: no method matching (::getfield(XGBoost, Symbol("#_setinfo#8")))(::Ptr{Nothing}, ::String, ::Array{Tuple{Vararg{String,N} where N},1})
Closest candidates are:
  _setinfo(::Ptr{Nothing}, ::String, ::Array{#s12,1} where #s12<:Number) at /Users/viral/.julia/packages/XGBoost/udvqz/src/xgboost_lib.jl:10
Stacktrace:
 [1] (::getfield(XGBoost, Symbol("##DMatrix#6#10")){getfield(XGBoost, Symbol("#_setinfo#8"))})(::Base.Iterators.Pairs{Symbol,Array{Tuple{Vararg{String,N} where N},1},Tuple{Symbol},NamedTuple{(:label,),Tuple{Array{Tuple{Vararg{String,N} where N},1}}}}, ::Type, ::SparseMatrixCSC{Float64,Int64}, ::Bool) at /Users/viral/.julia/packages/XGBoost/udvqz/src/xgboost_lib.jl:40
 [2] (::getfield(Core, Symbol("#kw#Type")))(::NamedTuple{(:label,),Tuple{Array{Tuple{Vararg{String,N} where N},1}}}, ::Type{DMatrix}, ::SparseMatrixCSC{Float64,Int64}, ::Bool) at ./none:0 (repeats 2 times)
 [3] makeDMatrix(::SparseMatrixCSC{Float64,Int64}, ::Array{Tuple{Vararg{String,N} where N},1}) at /Users/viral/.julia/packages/XGBoost/udvqz/src/xgboost_lib.jl:137
 [4] #xgboost#20(::Array{Tuple{Vararg{String,N} where N},1}, ::Array{Any,1}, ::Array{Any,1}, ::Array{Any,1}, ::Type, ::Type, ::Array{Any,1}, ::Base.Iterators.Pairs{Symbol,Int64,Tuple{Symbol,Symbol},NamedTuple{(:eta, :max_depth),Tuple{Int64,Int64}}}, ::typeof(xgboost), ::SparseMatrixCSC{Float64,Int64}, ::Int64) at /Users/viral/.julia/packages/XGBoost/udvqz/src/xgboost_lib.jl:147
 [5] (::getfield(XGBoost, Symbol("#kw##xgboost")))(::NamedTuple{(:label, :eta, :max_depth),Tuple{Array{Tuple{Vararg{String,N} where N},1},Int64,Int64}}, ::typeof(xgboost), ::SparseMatrixCSC{Float64,Int64}, ::Int64) at ./none:0
 [6] top-level scope at none:0