Testsuite: added support for testing runtime errors.

src/lib-sieve/sieve-interpreter.c

src/testsuite/Makefile.am

src/testsuite/ext-testsuite.c

src/testsuite/testsuite-common.c

src/testsuite/testsuite-common.h

src/testsuite/testsuite.c

src/testsuite/tst-test-execute.c

tests/execute/errors/action-conflicts.sieve

tests/execute/errors/action-duplicates.sieve