LibCore: Don't continue in forked child if exec() fails

Fixes SerenityOS#1854.

Libraries/LibCore/DesktopServices.cpp

@@ -55,7 +55,7 @@ bool spawn(String executable, String argument)
  if (child_pid == 0) {
  if (execl(executable.characters(), executable.characters(), argument.characters(), nullptr) < 0) {
  perror("execl");
- return false;
+ exit(1);
  }
  ASSERT_NOT_REACHED();
  }