Check for error from Azure when using the private-cluster feature (#270)

* Check for error from Azure

Move the error check for Azure earlier, so that a well defined error is
thrown on error instead of a JSONSyntax error.

The issue is that when Azure returns an error, like when there's an
issue with the access to the principal used. When this happens, the
stdout field will be an empty string, and the error message will be set.

* Restore check for deserialized exitCode

src/types/privatekubectl.ts

@@ -75,11 +75,18 @@ export class PrivateKubectl extends Kubectl {
             runOutput
          )}`
       )
+
+      if (runOutput.exitCode !== 0) {
+         throw Error(
+            `Call to private cluster failed. Command: '${kubectlCmd}', errormessage: ${runOutput.stderr}`
+         )
+      }
+
       const runObj: {logs: string; exitCode: number} = JSON.parse(
          runOutput.stdout
       )
       if (!silent) core.info(runObj.logs)
-      if (runOutput.exitCode !== 0 && runObj.exitCode !== 0) {
+      if (runObj.exitCode !== 0) {
          throw Error(`failed private cluster Kubectl command: ${kubectlCmd}`)
       }