Ensure CSR fallback only happens on actual errors

[artlowel]

References

Fixes issue introduced by #863

Description

This PR fixes an issue introduced by #863, which caused the error handler on the express server to be triggered when there were no errors.

Instructions for Reviewers

In #863 we switched to the native express error handler. But we noticed on dspace-demo that the error fallback happens for every request.

The reason is that it's not a dedicated error handler, but simply the next function in the chain that can make changes to the response. There's only an error if err is defined.

This PR fixes that issue by checking if there's an error, and if not, passing the data from the previous function (the angular app) through.

To test it, triggering a 404 (as suggested in #863) is not sufficient for causing an actual express error. So I suggest you simply add throw new Error('A test error'); to the constructor of ServerAppModule, in which case you should see Error in SSR, serving for direct CSR in the server output with a stack trace. And if you leave it out that fallback shouldn't trigger, so that message shouldn't be there.

Checklist

• My PR is small in size (e.g. less than 1,000 lines of code, not including comments & specs/tests), or I have provided reasons as to why that's not possible.
• My PR passes TSLint validation using yarn run lint
• My PR includes TypeDoc comments for all new (or modified) public methods and classes. It also includes TypeDoc for large or complex private methods.
• My PR passes all specs/tests and includes new/updated specs or tests based on the Code Testing Guide.
• If my PR includes new, third-party dependencies (in package.json), I've made sure their licenses align with the DSpace BSD License based on the Licensing of Contributions documentation.

[tdonohue]

👍 Looks good to me. Thanks @artlowel !