Ungraceful handling of libarchive missing symbol

When extractcode-libarchive isnt present, on openSUSE we see the following. A more graceful error indicating what to do would be helpful.
```py
[ 35s] src/extractcode/archive.py:29: in
[ 35s] from extractcode import libarchive2
[ 35s] :991: in _find_and_load
[ 35s] ???
[ 35s] :975: in _find_and_load_unlocked
[ 35s] ???
[ 35s] :671: in _load_unlocked
[ 35s] ???
[ 35s] /usr/lib/python3.8/site-packages/_pytest/assertion/rewrite.py:168: in exec_module
[ 35s] exec(co, module.__dict__)
[ 35s] src/extractcode/libarchive2.py:635: in
[ 35s] archive_reader = libarchive.archive_read_new
[ 35s] /usr/lib/python3.8/ctypes/__init__.py:386: in __getattr__
[ 35s] func = self.__getitem__(name)
[ 35s] /usr/lib/python3.8/ctypes/__init__.py:391: in __getitem__
[ 35s] func = self._FuncPtr((name_or_ordinal, self))
[ 35s] E AttributeError: /usr/bin/python3.8: undefined symbol: archive_read_new
```