- fixed compilation of POSIX targets

src/doomerrors.h:74:14: error: exception specification of overriding function is more lax than base version
src/posix/sdl/i_main.cpp:272:28: error: 'class std::exception' has no member named 'GetMessage'

src/doomerrors.h

@@ -71,7 +71,7 @@ public:
 		else
 			return NULL;
 	}
-	char const *what() const override
+	char const *what() const noexcept override
 	{
 		return m_Message;
 	}

src/posix/sdl/i_main.cpp

@@ -265,11 +265,11 @@ int main (int argc, char **argv)
 			fprintf (stderr, "%s\n", error.what ());
 
 #ifdef __APPLE__
-		Mac_I_FatalError(error.GetMessage());
+		Mac_I_FatalError(error.what());
 #endif // __APPLE__
 
 #ifdef __linux__
-		Linux_I_FatalError(error.GetMessage());
+		Linux_I_FatalError(error.what());
 #endif // __linux__
 
 		exit (-1);