Doing `throw <myExceptionVariable>` causes the stack trace information
to be rewritten (we don't want this). Wrapping in a new exception object
like `AggregateException` fixes that but also makes the printed
exception look messier.
Instead, simply return a bool to indicate if `HandleException()`
processed the the exception; if not, then we can rethrow from outside
the method.
Two separate duplicate checks have been introduced:
1. Within the same YAML file, YamlDotNet has been instructed to error on
duplicate keys.
2. Between different YAML files, custom logic enforces that there should
be no duplicate instance names.
Robert Dailey