Testsuite: completed support for error validation and added one test case.

src/lib-sieve/sieve-error.c

src/lib-sieve/sieve-error.h

src/testsuite/Makefile.am

src/testsuite/tests/compile/errors.svtest

src/testsuite/tests/compile/errors/header.sieve

src/testsuite/testsuite-common.c

src/testsuite/testsuite-common.h

src/testsuite/testsuite.c

src/testsuite/tst-test-compile.c

src/testsuite/tst-test-error.c