otptoken_yubikey.py: Removed traceback when package missing.

IPA should suggest user to install dependent packages instead
of throwing traceback. To work with IPA and Yubikey, packages
libyubikey(not in official RHEL repo) and libusb are required.

Resolves: https://pagure.io/freeipa/issue/6979
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>

ipaclient/plugins/otptoken_yubikey.py

@@ -131,6 +131,9 @@ class otptoken_add_yubikey(Command):
             raise NotFound(reason="No YubiKey found: %s" % e.strerror)
         except yubico.yubikey.YubiKeyError as e:
             raise NotFound(reason=e.reason)
+        except ValueError as e:
+            raise NotFound(reason=str(e) + ". Please install 'libyubikey' "
+                           "and 'libusb' packages first.")
 
         assert yk.version_num() >= (2, 1)