lib-sieve: sieve - Make sieve instance initialization fail if plugin fails to load

src/lib-sieve/sieve-plugins.c

src/lib-sieve/sieve-plugins.h

src/lib-sieve/sieve.c

src/lib-sieve/storage/ldap/sieve-ldap-storage.c

src/plugins/imapsieve/sieve-imapsieve-plugin.c

src/plugins/imapsieve/sieve-imapsieve-plugin.h

src/plugins/sieve-extprograms/sieve-extprograms-plugin.c

src/plugins/sieve-extprograms/sieve-extprograms-plugin.h