Add support for health diagnostic errors backwards compatibility

[vickyyyyyyy]

Context

With the new health diagnostics work, we have introduced a new error format for better user feedback:

Previous error message:

New error message:

What the problem is

The changes in core Grafana to allow the new format will be released in Grafana 8.
However, the new changes are not backwards compatible, due to the way we are structuring the backend response.

For the backend, we are now moving the old error message from CheckHealthResult.Message to CheckHealthResult.JSONDetails.verboseMessage which means for the frontend, we will need to move the old error message from testingStatus.message to testingStatus.details.verboseMessage.

Pre v8 expected backend response:

&backend.CheckHealthResult{
   Status:   backend.HealthStatusError,
   Message:  "Get \"http://asddasda/controller/rest/applications?output=json\": dial tcp: lookup asddasda on 194.168.4.100:53: no such host",
}

Post v8 expected backend response:

&backend.CheckHealthResult{
   Status:       backend.HealthStatusError,
   Message:      "Bad request (400)",
   JSONDetails:  []byte(`{
      "message":"Could not connect to AppDynamics. This usually happens when the URL is incorrect.",
      "verboseMessage":"Get "http://asddasda/controller/rest/applications?output=json": dial tcp: lookup asddasda on 194.168.4.100:53: no such host",
   }`),
}

Pre v8 expected Alert:

<Alert
   severity={testingStatus.status === 'error' ? 'error' : 'success'}
   title={testingStatus.message}
   aria-label={selectors.pages.DataSource.alert}
>
   {testingStatus.details?.message ?? null}
   {testingStatus.details?.verboseMessage ? (
      <details style={{ whiteSpace: 'pre-wrap' }}>{testingStatus.details?.verboseMessage}</details>
   ) : null}
</Alert>

Post v8 expected Alert:

<Alert
   severity={testingStatus.status === 'error' ? 'error' : 'success'}
   title={testingStatus.message}
   aria-label={selectors.pages.DataSource.alert}
/>

We need to find a way to support users running Grafana <8 because they would end up seeing: