Catch exception if passphrase dialog cancelled

As hopefully explained by comment

src/components/structures/auth/CompleteSecurity.js

@@ -40,12 +40,16 @@ export default class CompleteSecurity extends React.Component {
 
     onStartClick = async () => {
         const cli = MatrixClientPeg.get();
+        try {
             await accessSecretStorage(async () => {
                 await cli.checkOwnCrossSigningTrust();
             });
             this.setState({
                 phase: PHASE_DONE,
             });
+        } catch (e) {
+            // this will throw if the user hits cancel, so ignore
+        }
     }
 
     onSkipClick = () => {