Throw exception when pkcs11 lib is not found

[ricklambrechts]

Currently the application goes further after PKCS11LibFinder().find() and there is no lib found.

If there is no pcks11 lib found it will error, but it would be better to throw an exception beforehand.

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "nl-uzipoc-yubisign\app\wizard.py", line 72, in <module>
    mainWindow = MainWindow(pkcscls, acme, oidc_provider_url)
  File "nl-uzipoc-yubisign\app\wizard.py", line 42, in __init__
    self.wizard.addPage(SelectYubiKeyPage(mypkcs))
                        ~~~~~~~~~~~~~~~~~^^^^^^^^
  File "nl-uzipoc-yubisign\app\page\selectkey.py", line 48, in __init__
    yubikeys = self._get_yubikeys()
  File "nl-uzipoc-yubisign\app\page\selectkey.py", line 20, in _get_yubikeys
    for slot in self.pkcs.pkcs11.getSlotList():
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'getSlotList'

So it would be nice if we throw an exception here.

nl-uzipoc-yubisign/app/wizard.py

Lines 63 to 65 in 4a4cdc1

	# This will search default locations and fall back to the PYKCS11LIB environment variable
	pkcslib = PKCS11LibFinder().find()
	pkcscls = pkcs(pykcs11lib=pkcslib, yubikey_pin=yubikey_pin)

FYI, the default location for Windows with Yubico PIV tool is: _WINDOWS_DEFAULT_LOCATION = pathlib.Path("C:\\Program Files\\Yubico\\Yubico PIV Tool\\bin\\libykcs11.dll")