zscanner: add check for unclosed multiline block

src/common/errcode.c

src/common/errcode.h

src/zscanner/scanner.c

src/zscanner/scanner.rl

src/zscanner/test/cases/00-0_general.in

src/zscanner/test/cases/00-0_general.out

src/zscanner/test/cases/00-1_general.in

src/zscanner/test/cases/00-1_general.out

src/zscanner/test/processing.c