Don't panic if command is given in interactive mode, give a nice error
message instead.
Before:
$ ./bin/etcdctl watch -i
<hit return>
panic: runtime error: index out of range
goroutine 1 [running]:
etcdctl/ctlv3/command.watchInteractiveFunc(...)
etcd/etcdctl/ctlv3/command/watch_command.go:104 ...
After:
$ ./bin/etcdctl watch -i
<hit return>
Invalid command: (watch and progress supported)
foo
Invalid command foo (only support watch)
36d7acf330