Failing webpack/declaration build does not cause cli process to exit with non-zero code

We noticed when the webpack build fails or the typescript declaration emitting step fails, the cli process does not exit with a non-zero code. This is problematic in a CI context as the CI pipeline will continue running the next steps in the job/next job as it doesn't detect the failure.

The following updates should probably be made:

- Catch errors thrown by `buildDeclaration`
- If an error is caught by `buildDeclaration` or the `errors` variable has a truthy value, add code to call `process.exit` with a non-zero code.