Use julia parser in completion - take 2

Alternative version to #6344. This one does not touch the parser or any of the code in src/, and is generally much cleaner. The weak spot is of course that it determines the reason for incompleteness from the error message, but I think that's acceptable for the time being, considering that 1) it's not everyday that error messages are changed 2) the advantages for the parser are a much cleaner and robust code 3) any alternative AFAICT requires changes to the parser which range from "superficial but dissatisfying" to "deep but substantial (i.e. long-term)". So I think it's worth it for now.
Opinions?