Improved argument error reporting.

TODO

src/lib-sieve/cmd-redirect.c

src/lib-sieve/cmd-require.c

src/lib-sieve/ext-encoded-character.c

src/lib-sieve/ext-envelope.c

src/lib-sieve/plugins/body/tst-body.c

src/lib-sieve/plugins/imapflags/ext-imapflags-common.c

src/lib-sieve/plugins/include/cmd-import.c

src/lib-sieve/plugins/include/cmd-include.c

src/lib-sieve/plugins/regex/mcht-regex.c

src/lib-sieve/plugins/relational/ext-relational-common.c

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

src/lib-sieve/plugins/variables/cmd-set.c

src/lib-sieve/plugins/variables/ext-variables-arguments.c

src/lib-sieve/sieve-commands.h

src/lib-sieve/sieve-comparators.c

src/lib-sieve/sieve-match-types.c

src/lib-sieve/sieve-match-types.h

src/lib-sieve/sieve-validator.c

src/lib-sieve/tst-address.c