tarantoolctl: review fixes for gh-672
tarantoolctl: review fixes for gh-672

* Remove useless syntax checking:

   `echo "2+2" | tarantoolctl eval example` now works as well as
   `echo "return 2+2" | tarantoolctl eval example`

* Don't read file from the disk twice
* Improve error handling
* Streamline isatty() usage

rtsisyk/tarantool,rtsisyk/tarantool,mejedi/tarantool,mejedi/tarantool,mejedi/tarantool,mejedi/tarantool,rtsisyk/tarantool,rtsisyk/tarantool