panic during save if the cmd line argument is a dir

[icanrealizeum]

Pressing Ctrl+S after running iota with an existing dir arg

$ cargo run --release ./src/
!! executing '/home/z/build/1nonpkgs/cargo/cargo//target/x86_64-unknown-linux-gnu/release//cargo' with args: 'run --release ./src/'
     Running `target/release/iota ./src/`
thread 'main' panicked at 'file error: Not a directory (os error 20)', src/iota/view.rs:341
stack backtrace:
   1:     0x55fa0859680f - std::sys::backtrace::tracing::imp::write::hc6294d1e0e9044ff
   2:     0x55fa0859a40b - std::panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::h695d4ae4d161909f
   3:     0x55fa0859a08f - std::panicking::default_hook::h369905e6af8b368f
   4:     0x55fa0858bfde - std::panicking::rust_panic_with_hook::h5fd54c69674f2610
   5:     0x55fa0859a651 - std::panicking::begin_panic::h8be16b42173b5799
   6:     0x55fa0858cd9a - std::panicking::begin_panic_fmt::h65c8bf73bb1f86b1
   7:     0x55fa0851c258 - iota::view::View::try_save_buffer::h370bc8f2b36346a9
   8:     0x55fa084dc5e2 - iota::main::h1db1321deeb66bc3
   9:     0x55fa08599cc8 - std::panicking::try::call::h23d0d9523f424720
  10:     0x55fa085a1afb - __rust_try
  11:     0x55fa085a1a9e - __rust_maybe_catch_panic
  12:     0x55fa0859976e - std::rt::lang_start::h7023097bb9b24642
  13:     0x7f99539a4740 - __libc_start_main
  14:     0x55fa084da058 - _start
  15:                0x0 - <unknown>
error: Process didn't exit successfully: `target/release/iota ./src/` (exit code: 101)