Better error message when forming a DataFrame from a vector of dictionaries with missing data.

[LilithHafner]

julia> DataFrame([Dict(:X => 2, :Y => 4), Dict(:X => 7, :Y => 49)])
2×2 DataFrame
 Row │ X      Y     
     │ Int64  Int64 
─────┼──────────────
   1 │     2      4
   2 │     7     49

julia> DataFrame([Dict(:X => 2, :Y => 4), Dict(:X => 7)])
ERROR: KeyError: key :Y not found
Stacktrace:
  [1] getindex(h::Dict{Symbol, Int64}, key::Symbol)
    @ Base ./dict.jl:510
  [2] getcolumn
    @ ~/.julia/packages/Tables/NSGZI/src/Tables.jl:143 [inlined]
  [3] eachcolumns
    @ ~/.julia/packages/Tables/NSGZI/src/utils.jl:127 [inlined]
  [4] __buildcolumns(rowitr::Vector{…}, st::Int64, sch::Tables.Schema{…}, columns::Tuple{…}, rownbr::Int64, updated::Base.RefValue{…})
    @ Tables ~/.julia/packages/Tables/NSGZI/src/fallbacks.jl:174
  [5] _buildcolumns(rowitr::Vector{…}, row::Dict{…}, st::Int64, sch::Tables.Schema{…}, columns::Tuple{…}, updated::Base.RefValue{…})
    @ Tables ~/.julia/packages/Tables/NSGZI/src/fallbacks.jl:198
  [6] buildcolumns
    @ ~/.julia/packages/Tables/NSGZI/src/fallbacks.jl:227 [inlined]
  [7] _columns
    @ ~/.julia/packages/Tables/NSGZI/src/fallbacks.jl:265 [inlined]
  [8] columns
    @ ~/.julia/packages/Tables/NSGZI/src/fallbacks.jl:258 [inlined]
  [9] DataFrame(x::Vector{Dict{Symbol, Int64}}; copycols::Nothing)
    @ DataFrames ~/.julia/packages/DataFrames/58MUJ/src/other/tables.jl:57
 [10] DataFrame(x::Vector{Dict{Symbol, Int64}})
    @ DataFrames ~/.julia/packages/DataFrames/58MUJ/src/other/tables.jl:48
 [11] top-level scope
    @ REPL[8]:1
Some type information was truncated. Use `show(err)` to see complete types.

It would be nice if

1. The error message was more clear about the fact that the dictionaries do not all share the same keyset
2. There were an option (hinted to in the error message) to make missing entries cause missing entries in the dataframe rather than an error

From Logo on zulip link

[jariji]

Dup of JuliaData/Tables.jl#352 which I should have posted in the Zulip thread.

[LilithHafner]

Interesting that I'm getting a "key not found" error and you're getting a "has no field" error. Closing in favor of JuliaData/Tables.jl#352