lib-sieve: sieve-storage - Use int for sieve_storage_get_script() status result

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

src/lib-sieve/plugins/include/ext-include-binary.c

src/lib-sieve/sieve-script.c

src/lib-sieve/sieve-storage.c

src/lib-sieve/sieve-storage.h