Implement 12-bit-specific error/warn/trace macros The macros in jerror.h refer to j_common_ptr, so it is unfortunately necessary to introduce a 12-bit-specific version of that header file (j12error.h) with 12-bit specific ERREXIT*(), WARNMS*(), and TRACEMS*() macros. (The message table is still shared between 8-bit and 12-bit implementations.) Fixes #607