Sieve plugin: simplified system error handling.

src/lib-sieve/sieve-error.c

src/lib-sieve/sieve-error.h

src/plugins/lda-sieve/Makefile.am

src/plugins/lda-sieve/lda-sieve-log.c

src/plugins/lda-sieve/lda-sieve-log.h

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