exit if wrong filename is provided

TODO: for qml maybe it can url but QDeclarativeView.setSource() ends
up in segfault if source file is wrong

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[cutes] fix error passing

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>