Error when launched from a non-existent directory

Example:

```
$ mkdir foo
$ cd foo
$ rmdir ../foo
```

Running `ntfy send message` or, more annoyingly when shell integration is enabled, just pressing Enter, causes this:

```
Traceback (most recent call last):
File "/Users/mohamed/Library/Python/3.7/bin/ntfy", line 6, in
from ntfy.cli import main
File "/Users/mohamed/Library/Python/3.7/lib/python/site-packages/ntfy/__init__.py", line 12, in
_cwd = getcwd()
FileNotFoundError: [Errno 2] No such file or directory
```