Sandbox environment warns user when providing default search responses

[Dunmail]

Is your feature request related to a problem? Please describe.
The Sandbox environment returns an empty Bundle in response to Patient searches where there is no predefined response. This leads to misleading behaviour. For example:
Patient?family=Sm* returns 2 matches
Patient?family=Smi* returns 0 matches
Patient?family=Smith returns 1 match

Describe the solution you'd like
Include an OperationOutcome within the default Bundle to indicate that there is no mock so that consumers understand that the response is a limitation with the environment rather than a problem with the processing of the request.

Describe alternatives you've considered
The case where there is no predefined response is handled in patient-searcher.js ln68, as follows:

delete response.entry 

Instead, add a warning outcome:

response.entry.push({
  search: {
    mode: 'outcome'
  },
  resource: {
    resourceType: 'OperationOutcome',
    issue: [
      {
        severity: 'warning',
        code: 'not-supported',
        diagnostics: 'This mock endpoint has no example response for this combination of search parameters'
      }
    ]
  }
})

Additional context
I would have provided a pull request but couldn't work out how to generate the branch name in accordance with the guidelines!