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

release/installer.nsi.in

@@ -109,6 +109,7 @@ Section "@CMAKE_PROJECT_NAME@ SDK for @INST_PLATFORM@ (required)"
 	File "@CMAKE_CURRENT_SOURCE_DIR@\jpeglib.h"
 !ifdef 12BIT
 	File "@CMAKE_CURRENT_SOURCE_DIR@\jpeg12lib.h"
+	File "@CMAKE_CURRENT_SOURCE_DIR@\j12error.h"
 !endif
 	File "@CMAKE_CURRENT_SOURCE_DIR@\turbojpeg.h"
 	SetOutPath $INSTDIR\doc

release/rpm.spec.in

@@ -230,6 +230,7 @@ rm -rf $RPM_BUILD_ROOT
 %{_includedir}/jpeglib.h
 %if "%{_with_12bit}" == "1"
 	%{_includedir}/jpeg12lib.h
+	%{_includedir}/j12error.h
 %endif
 %if "%{_with_turbojpeg}" == "1"
 	%{_includedir}/turbojpeg.h