Set output for the error message

In the event vault-action throws an error, the error message is
sent to the workflow log when with core.setFailed, but that output
is not accessible to other steps in the workflow.

This change sets an output for the error message, called errorMessage,
than can be read by other steps within the workflow and parsed.

src/entry.js

@@ -5,6 +5,7 @@ const { exportSecrets } = require('./action');
     try {
         await core.group('Get Vault Secrets', exportSecrets);
     } catch (error) {
+        core.setOutput("errorMessage", error.message);
         core.setFailed(error.message);
     }
 })();