improve error message when column is not found

[bkamins]

Following discussion on Slack.

After the change:

julia> df = DataFrame()
0×0 DataFrame

julia> df.a
ERROR: ArgumentError: column name "a" not found in the data frame since it has no columns

julia> df = DataFrame(a=1, b=2, c=3)
1×3 DataFrame
 Row │ a      b      c     
     │ Int64  Int64  Int64 
─────┼─────────────────────
   1 │     1      2      3

julia> df.d
ERROR: ArgumentError: column name "d" not found in the data frame; existing most similar names are: "a", "b" and "c"

I use string quoting not : and backticks as suggested on Slack as we allow string as column name.

[bkamins]

Thank you!