[SV 40236] Handle included file open failures properly.

* read.c (eval_makefile): Set deps->error if we discovered any
error reading makefiles, and set NONEXISTENT_MTIME so we know
it needs to be rebuilt.
* main.c (main): Clean up management of makefile_mtimes.
* tests/scripts/features/include: Add open failure testcases.
3 files changed