104085 r=cavin sr=bienvenu Add NS_MSG_CUSTOM_HEADERS_OVERFLOW error

mailnews/base/public/msgCore.h

@@ -157,6 +157,7 @@ NS_ERROR_GENERATE_FAILURE(NS_ERROR_MODULE_MAILNEWS, value)
              Message compose use the same error code space than other mailnews modules. To avoid any
              conflict, I reserve values between 12500 and 12999 for it.
 */
+#define NS_MSG_CUSTOM_HEADERS_OVERFLOW NS_MSG_GENERATE_FAILURE(28) //when num of custom headers exceeds 50
 #define NS_MSGCOMP_ERROR_BEGIN	12500
 /* NS_ERROR_NNTP_NO_CROSS_POSTING lives here, and not in nsMsgComposeStringBundle.h, because it is used in news and compose. */
 #define NS_ERROR_NNTP_NO_CROSS_POSTING NS_MSG_GENERATE_FAILURE(12554)