Revised positional argument checking and fixed the validator's error handling.

sieve/errors/address-errors.sieve

sieve/errors/header-errors.sieve

src/lib-sieve/cmd-redirect.c

src/lib-sieve/cmd-require.c

src/lib-sieve/ext-envelope.c

src/lib-sieve/ext-fileinto.c

src/lib-sieve/ext-reject.c

src/lib-sieve/plugins/subaddress/subaddress.sieve

src/lib-sieve/plugins/vacation/ext-vacation.c

src/lib-sieve/sieve-ast.c

src/lib-sieve/sieve-ast.h

src/lib-sieve/sieve-common.h

src/lib-sieve/sieve-validator.c

src/lib-sieve/sieve-validator.h

src/lib-sieve/tst-address.c

src/lib-sieve/tst-exists.c

src/lib-sieve/tst-header.c

src/lib-sieve/tst-size.c