Multiple TO version in ALTER EXTENSION UPDATE

While reading I noticed that we allow multiple TO <version> in ALTER EXTENSION
UPDATE, and defer throwing a syntax error until command processing. Is there a
reason for deferring and not handling it in gram.y directly as in the attached
patch since it is in fact a syntax error? It yields a different error message
to the user, but makes for easier to read code (IMH-and-biased-O).

cheers ./daniel