Revert "cpp: do not pass #error into output stream, warn about it and exit non-0"

[citrus-it]

This reverts commit ee5767d.

This commit seems wrong.

With it, if you compile cpp without EXIT_ON_ERROR defined, cpp just doesn't understand #error at all and outputs things like this during a gate build:

.../usr/include/sys/asm_linkage.h: 102: undefined control

With EXIT_ON_ERROR set, a gate build fails with:

  sys/asm_linkage.h: 102: #error      "inconsistent shift constants"

I think it's wrong to have a switch with toggles between "#error not understood" and "#error acted upon" and we should revert to "#error passed through to output stream" (as per OmniOS and SmartOS today) and "#error acted upon" - and then we can work on making the latter work with a gate build.