Better concat_rows error messages

[billylanchantin]

Fixes: #1023

Example:

df0 = DF.new([x: [1, 2, 3], y: ["g", "h", nil]])
df1 = DF.new([z: [4, 5, 6]])
DF.concat_rows(df0, df1)

dataframes must have the same columns

* dataframe 0 has these columns not present in dataframe 1:

    ["x", "y"]

* dataframe 1 has these columns not present in dataframe 0:

    ["z"]

[billylanchantin]

@josevalim I'm aiming to emulate the new type error messages in Elixir with the indentations and newlines. If you would, please review with that in mind.

We can maybe skip the ANSI colors though since that'd be a big departure from our other error messages.

[josevalim]

Beautiful!