warnings: fix early exit of warnings treated as errors

Treating warnings as errors caused Bison to exit earlier than needed, making it
hide warnings that would have been printed had -Werror not been set.

Also, fix a bug that caused some context information of errors to not be
shown.

* src/complain.c (complaint_issued): Rename as...
(complaint_status): This, and change its type from boolean to
* src/complain.h (err_status): This, new enumeration.
* src/main.c (main): Adjust (only finish early if an actual complaint was
risen, not a mere warning treated an error).
* src/reader.c: Adjust.