Helpful warning when you enter `exit`

[rtfeldman]

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Some other REPLs (notably irb) are exited by entering exit at the prompt. Currently if you do that in elm-repl you get this:

> exit

Error in repl-temp-000.elm:

Error on line 3, column 3 to 7:
Could not find variable 'exit'.

It would be nice if elm-repl detected this case (probably as well as quit and :q), and in addition to throwing the usual error, also helpfully suggested that perhaps the user wants to type :exit

[brobrts]

It would be nice if this was there when you run $ elm-repl --help

[opsb]

google pointed me here to find out how to quit elm-repl, so in a way half the job is done ;)