Major rework of the multiscript support for better error handling.

src/lib-sieve/sieve-error-private.h

src/lib-sieve/sieve-error.c

src/lib-sieve/sieve-result.c

src/lib-sieve/sieve-result.h

src/lib-sieve/sieve-script.c

src/lib-sieve/sieve.c

src/lib-sieve/sieve.h

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

src/sieve-tools/sieve-test.c