[Reporting/CSV] Insert error message into contents if export is empty (#175852)

## Summary

Closes https://github.com/elastic/kibana/issues/174659

### Release note
Updated CSV export to insert error messages into the contents if the
export results in an empty file due to an error.

### Checklist

Delete any items that are not applicable to this PR.

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

packages/kbn-generate-csv/src/generate_csv.test.ts

@@ -1280,6 +1280,7 @@ describe('CsvGenerator', () => {
   });
 
   it('handles unknown errors', async () => {
+    const streamWriteSpy = jest.spyOn(stream, 'write');
     mockDataClient.search = jest.fn().mockImplementation(() => {
       throw new Error('An unknown error');
     });
@@ -1300,6 +1301,7 @@ describe('CsvGenerator', () => {
       mockLogger,
       stream
     );
+
     await expect(generateCsv.generateData()).resolves.toMatchInlineSnapshot(`
       Object {
         "content_type": "text/csv",
@@ -1317,6 +1319,18 @@ describe('CsvGenerator', () => {
         ],
       }
     `);
+    expect(streamWriteSpy.mock.calls).toMatchInlineSnapshot(`
+      Array [
+        Array [
+          "",
+        ],
+        Array [
+          "
+      \\"Encountered an unknown error: An unknown error\\"
+      \\"Encountered an error with the number of CSV rows generated from the search: expected rows were indeterminable, received 0.\\"",
+        ],
+      ]
+    `);
   });
 
   describe('error codes', () => {

packages/kbn-generate-csv/src/generate_csv.ts

@@ -401,6 +401,24 @@ export class CsvGenerator {
       }
     }
 
+    if (this.csvRowCount === 0) {
+      if (warnings.length > 0) {
+        /*
+         * Add the errors into the CSV content. This makes error messages more
+         * discoverable. When the export was automated or triggered by an API
+         * call or is automated, the user doesn't necesssarily go through the
+         * Kibana UI to download the export and might not otherwise see the
+         * error message.
+         */
+        logger.info('CSV export content was empty. Adding error messages to CSV export content.');
+        // join the string array and putting double quotes around each item
+        // add a leading newline so the first message is not treated as a header
+        builder.tryAppend('\n"' + warnings.join('"\n"') + '"');
+      } else {
+        logger.info('CSV export content was empty. No error messages.');
+      }
+    }
+
     return {
       content_type: CONTENT_TYPE_CSV,
       csv_contains_formulas: this.csvContainsFormulas && !escapeFormulaValues,