lib-sieve: sieve-extensions - Notify caller of sieve_extensions_set_string() about extension load failures

Since the script storage code is widely engrained in the Pigeonhole sources,
this is a very large commit that cannot be reduced further into smaller
preparatory bits.

For now, these yield the same result.

It doesn't match Dovecot convention.

Adds status parameter to sieve_result_execute().

Avoids unnecessary pointer parameter.