C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/exceptions.h

@@ -17,8 +17,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
 
-#ifndef EXCEPTIONS_HEADER
-#define EXCEPTIONS_HEADER
+#pragma once
 
 #include <exception>
 #include <string>
@@ -129,6 +128,3 @@ public:
 		BaseException(s)
 	{}
 };
-
-#endif
-