plugins: doveadm-sieve: doveadm-sieve-sync - Refactor error handling in sieve_attribute_set_active()

Makes code easier on the eyes.

For consistency with managesieve.

Also make code cleaner and consistent.

For consistency with managesieve_capabilities_dump().

lib-sieve: plugins: enotify: ext-enotify-common - Rework error handling to match extension API changes

Was handled in each extension before, which is useless.

lib-sieve: plugins: environment: Use int for sieve_ext_environment_require_extension() status result