[BUG] Multiple REST APIs ignore the FlowFrameworkException status

[dbwiddis]

What is the bug?

Multiple RestActions catch the FlowFrameworkException but then ignore its status using ExceptionsHelper:

FlowFrameworkException ex = new FlowFrameworkException(exception.getMessage(), ExceptionsHelper.status(exception));
XContentBuilder exceptionBuilder = ex.toXContent(channel.newErrorBuilder(), ToXContent.EMPTY_PARAMS);
channel.sendResponse(new BytesRestResponse(ex.getRestStatus(), exceptionBuilder));

How can one reproduce the bug?

1. Call an impacted REST API (e.g., GET workflow)
2. Observe the correct exception message but an incorrect 500 RestStatus.

What is the expected behavior?

The RestStatus from the FlowFrameworkException is returned.

Do you have any additional context?

I've addressed this in PR #271 by changing the first line above to:

FlowFrameworkException ex = exception instanceof FlowFrameworkException
    ? (FlowFrameworkException) exception
    : new FlowFrameworkException(exception.getMessage(), ExceptionsHelper.status(exception));

That needs to be copied over to all the other REST handlers.

Suggest this bug is addressed at the same time as #292 and #274