Do not treat canceled imports as failed

showError() must only be called for failed imports, not for canceled
imports. Adapt importFailed() helper and add importWasCanceled() helper
for better handling of both cases.

In keyCacheUpdated() improve the heuristric to check if the user
canceled all imports by checking if at least one import per id (i.e.
per file in case of import from files) was canceled. Don't show a
summary if no certificates were considered for import and the user
seems to have canceled all imports, e.g. in case of an encrypted
PKCS#12 import with canceled passphrase entry.

• GnuPG-bug-id: T5713