Contact failure: Add better icon and remove technical error message

...iarproject/bramble/keyagreement/KeyAgreementTaskImpl.java

briar-android/artwork/qr_code_error.svg

...t/briar/android/keyagreement/ContactExchangeActivity.java

...ar/android/keyagreement/ContactExchangeErrorFragment.java

...ject/briar/android/keyagreement/KeyAgreementActivity.java

briar-android/src/main/res/drawable/qr_code_error.xml

...d/src/main/res/layout/fragment_error_contact_exchange.xml

briar-android/src/main/res/values/strings.xml