Raise ArgumentError in Keyword.keys when list is not a keyword list

[eksperimental]

Previously it would return something like this:

** (FunctionClauseError) no function clause matching in anonymous fn/1 in Keyword.keys/1

The following arguments were given to anonymous fn/1 in Keyword.keys/1:

    # 1
    {"fetch/2", {:fetch, 2}}

Closes #10010, and it improves 3fd68ef

[josevalim]

Suggested change

	"expected a keyword list, but an element in the list is not a keyword; got: #{
	inspect(element)
	}"
	"expected a keyword list, but an element in the list is not a two-element tuple with atom key, " <>
	"got: #{inspect(element)}"

[eksperimental]

Implement your suggestion with the addition of the article "an".

I am not 100% sure on the accuracy of the message though, as tuples don't have keys.
How about something like this?

expected a keyword list, but an element in the list is not a two-element tuple with its first element being an atom; got:

[michalmuskala]

Raising deep from within the map call might result in a pretty poor error message with the whole printable stacktrace being only the map calls. I would suggest throwing from within the map, catching outside and re-raising with a stacktrace of the Keyword.keys/1 directly

[josevalim]

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