[osh/cmd_parse] Produce a more useful error message if a dangling do is detected

[PossiblyAShrub]

I was experimenting with producing error messages that try to guess at the user's intent and then produce more specific error messages. In this pr, I've added specific error messaging for errors like the following:

for x in a b c do
  echo $x
done

Before my changes we get the following error message:

        echo $x                                                                                                                               
        ^~~~                                                                                                                                  
test.sh:2: Expected word type Id.KW_Do, got Id.Word_Compound

With my changes:

  for x in a b c do                                                                                                                           
                 ^~                                                                                                                           
test.sh:1: Expected a semi-colon or newline before the "do". 

This PR is more of a proof-of-concept, a more thought out PR might generalize this check to while loops.