Imapflags: improved handling of invalid flags.

TODO

src/lib-sieve/cmd-redirect.c

src/lib-sieve/plugins/imapflags/cmd-addflag.c

src/lib-sieve/plugins/imapflags/cmd-removeflag.c

src/lib-sieve/plugins/imapflags/cmd-setflag.c

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

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

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

src/lib-sieve/plugins/imapflags/imapflags-errors.sieve

src/lib-sieve/plugins/imapflags/imapflags.sieve

src/lib-sieve/plugins/imapflags/tag-flags.c

src/lib-sieve/plugins/imapflags/tst-hasflag.c

src/lib-sieve/sieve-commands.h

src/lib-sieve/sieve-result.c

src/lib-sieve/sieve-result.h