HTTP Credentials: Read the password from the old location if not found.

Earlier clients used QtKeychain without a QSettings object, which made
QtKeychain to write the password encrypted into a settings default
location, ie. the registry under windows.

If we can not find a password at the new location it is tried to read
the password from the old default location once. That makes people
happy in migration scenarios.
HTTP Creds: In case of Keychain error, open the interact password dialog

But in case the user clicks cancel in the interactive dialog, invalidate
the credentials. Emit fechted() also in error case.