Show error in case of wrong QR Code

[cornelinux]

This will be obsolete when implementing #10

If a wrong QR Code is scanned, nothing happens.
Sometimes I try to scan a HOTP QRCode. Nothing happens. I am sitting and waiting and wondering, till I realize it was not TOTP.
A popup telling me "This is no valid QR Code. We only support TOTP" would be great.

[mueller-ma]

This repo seems to be unmaintained, but there is a active fork, which is also in F-Droid: https://github.com/kckrinke/onetimepad