Make hx properly handle being passed invalid paths

[poliorcetics]

Fixes #26.
Fixes #23.

With this PR hx is now able to properly report an error when it is given a directory, an invalid filename or a broken pipe:

# All those cases crashed previously.

❯ ./target/release/hx filename-missing
error = "No such file or directory (os error 2)"

❯ ./target/release/hx src
error = "Is a directory (os error 21)"

❯ ./target/release/hx symlink_to_dir
error = "Is a directory (os error 21)"

❯ dd if=/dev/random bs=512 count=10 | RUST_BACKTRACE=1 ./target/debug/hx | head -n 10
10+0 records in
10+0 records out
5120 bytes transferred in 0.000036 secs (142217460 bytes/sec)
0x000000: 0x6c 0xdc 0x4f 0x43 0xfd 0x5e 0xb3 0xa7 0x79 0xaa l.OC.^..y.
0x00000a: 0x6b 0x2c 0x81 0x47 0x00 0x10 0xa6 0x40 0xb5 0x6a k,.G...@.j
0x000014: 0x4d 0x31 0x5b 0x11 0x15 0x71 0xb8 0xa1 0xff 0x91 M1[..q....
0x00001e: 0x2e 0xaa 0x69 0x77 0x52 0x2a 0x20 0xa6 0x52 0xe5 ..iwR* .R.
0x000028: 0x41 0x2b 0x83 0x55 0x21 0x41 0x33 0xae 0xf8 0x38 A+.U!A3..8
0x000032: 0xba 0x74 0xfe 0x23 0xc2 0xc9 0xf5 0x3d 0x28 0x8e .t.#...=(.
0x00003c: 0xbc 0x0e 0xe1 0xa3 0xbf 0xda 0x81 0x92 0xc6 0xe9 ..........
0x000046: 0x9f 0xb7 0x13 0x72 0x79 0xd3 0x1e 0x4c 0xd8 0x5d ...ry..L.]
0x000050: 0x85 0x36 0x91 0x9f 0x13 0x7e 0x55 0x5f 0x7b 0xa6 .6...~U_{.
0x00005a: 0x23 0xe7 0xed 0x54 0x47 0x9a 0x1c 0x54 0x91 0x85 #..TG..T..
error = "Broken pipe (os error 32)"

[sitkevij]

pointing PR #27 to develop branch for integration testing.