Prevent index out of bounds error when reading empty log file

When trying to run utt hello after manually deleting all the content
in the report-file. I did this to start over.

The error comes from add_entry.py on line 26. If the report-file is
empty you can't put the cursor on position -1

```python
with open(filename, "rb+") as file:
    file.seek(-1, os.SEEK_END)  # if the file is empty, position -1 will cause OSError
   ...
```

I fixed it by catching the exception and letting the error pass,
whilst setting;

prepend_new_line = False

to pass integration-testing I also had to let errno.ENOENT
(FileNotFound) pass without raising an Exception - If the file doesn't
exist it gets created.

Issue: larose#19

utt/components/add_entry.py

@@ -28,6 +28,10 @@ def _append_line_to_file(filename, line, insert_new_line_before):
  file.seek(-1, os.SEEK_END)
  last_char = file.read(1)
  prepend_new_line = last_char != b"\n"
+ except OSError as os_err:
+ if os_err.errno not in [errno.EINVAL, errno.ENOENT]:
+ raise
+ prepend_new_line = False
  except EnvironmentError as exc:
  if exc.errno != errno.ENOENT:
  raise