Proper error message for invalid "SELECT col AS * FROM ds" query

[FinchPowers]

Blog post about MLDB Hacktoberfest 2016

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Running the query

SELECT col AS * FROM ds

is illegal. It currently returns a non useful message with a status code 500 (server error). It should instead return a useful message with a status code 400 (client error).

Task

Mldb1788SelectColAsStarErr test must pass without "expectedFailure".