Fix for FileNotFoundError when mercurial or git is not installed

[arunelias]

Exception occurs when mercurial or git is not installed, subprocess.CalledProcessError exception is not raised by the subprocess.check_output() command. [WinError 2] The system cannot find the file specified exception is raised instead.
Django Version: 4.0.5
Python Version: 3.9.13

Steps to reproduce

Mercurial or git is not installed
Add django_cachekiller to your INSTALLED_APPS in your settings.py
Then use the cdnstatic tag in the template

Exception

FileNotFoundError at /
[WinError 2] The system cannot find the file specified
Request Method: GET
Request URL: http://localhost:8000/
Django Version: 4.0.5
Exception Type: FileNotFoundError
Exception Value:
[WinError 2] The system cannot find the file specified
Exception Location: D:\Python39\lib\subprocess.py, line 1420, in _execute_child
Python Executable: D:\Python39\python.exe
Python Version: 3.9.13

Template error:
39 :

File "D:\Python39\lib\site-packages\django_cachekiller\templatetags\cdnstaticfiles.py", line 92, in render
qs = 'tag=' + refcache.get_ref()
File "D:\Python39\lib\site-packages\django_cachekiller\templatetags\cdnstaticfiles.py", line 69, in get_ref
ref = self.get_hg_tip()
File "D:\Python39\lib\site-packages\django_cachekiller\templatetags\cdnstaticfiles.py", line 53, in get_hg_tip
ref = self._run_cmd(['hg', 'heads', '--template', '"{node}"'])
File "D:\Python39\lib\site-packages\django_cachekiller\templatetags\cdnstaticfiles.py", line 26, in _run_cmd
return subprocess.check_output(cmd, stderr=subprocess.STDOUT)
File "D:\Python39\lib\subprocess.py", line 424, in check_output
return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
File "D:\Python39\lib\subprocess.py", line 505, in run
with Popen(*popenargs, **kwargs) as process:
File "D:\Python39\lib\subprocess.py", line 951, in init
self._execute_child(args, executable, preexec_fn, close_fds,
File "D:\Python39\lib\subprocess.py", line 1420, in _execute_child
hp, ht, pid, tid = _winapi.CreateProcess(executable, args,

Exception Type: FileNotFoundError at /
Exception Value: [WinError 2] The system cannot find the file specified

[meeb]

Good catch, thanks!

[meeb]

Your PR is in the new v0.8.2 release. Thanks.