'seq' should return 1, not exit 1, on bad arglist

Try running ```seq a```.  Since seq is a fish function, not a program, it calling "exit" causes the entire fish shell to exit.  This is a problem and probably not intended behavior.  This commit fixes it.  (No other fish functions use exit, except delete-or-exit, which is obviously intentional, and tests/test.fish, which I'm not sure about, but which appears to work.)