Add nice error message for zero-length files.

[brian6932]

So with c7b7717, the feature does work, however I noticed 3 issues:

1. It seems to end early, calculating the total keypresses incorrectly
2. When a typo is made in a word, instead of the word being turned red, the whole text becomes red
3. Pressing esc, then r causes a panic, this should probably just restart the current prompt

   thread 'main' panicked at 'index out of bounds: the len is 0 but the index is 0', src\ui.rs:98:26
   stack backtrace:
      0:     0x7ff7846c74b2 - <unknown>
      1:     0x7ff7846deaeb - <unknown>
      2:     0x7ff7846c36ca - <unknown>
      3:     0x7ff7846c71fb - <unknown>
      4:     0x7ff7846c9949 - <unknown>
      5:     0x7ff7846c95cb - <unknown>
      6:     0x7ff7846ca078 - <unknown>
      7:     0x7ff7846c9f6e - <unknown>
      8:     0x7ff7846c7e99 - <unknown>
      9:     0x7ff7846c9c20 - <unknown>
     10:     0x7ff7846f0ae5 - <unknown>
     11:     0x7ff7846f0c77 - <unknown>
     12:     0x7ff78463ec9d - <unknown>
     13:     0x7ff78463f8fe - <unknown>
     14:     0x7ff78463bc52 - <unknown>
     15:     0x7ff78462a976 - <unknown>
     16:     0x7ff78462a9a1 - <unknown>
     17:     0x7ff7846be0ce - <unknown>
     18:     0x7ff78463c8fc - <unknown>
     19:     0x7ff7846e5ae0 - <unknown>
     20:     0x7ffe279f117e - BaseThreadInitThunk
     21:     0x7ffe289042db - RtlUserThreadStart

[max-niederman]

I suspect that this is because the file is 0 words long, causing ttyper to error because it assumes there's always a current word.

I'll address this in v2 (#83), but if anyone is interesting in implementing a nice error message for this in v1, that would be a good first issue.