training/tensorflow: ignore content on stderr if printing metrics pro…

…duced something

Tensorflow 1.13 (required to use Python 3.7) emiths a DeprecationWarning
when genie-print-metrics is used, because the summary iterator
uses deprecated API internally (sad).

lib/training/tensorflow.js

@@ -145,12 +145,15 @@ module.exports = class TensorflowTrainingJob extends BaseTrainingJob {
             `--${this.config.eval_early_stopping_metric_minimize ? '' : 'no'}eval_early_stopping_metric_minimize`,
         ]);
 
-        if (stderr)
-            throw new Error(stderr);
-
-        this.metrics = {};
         stdout = stdout.trim();
+        if (!stdout) {
+            if (stderr)
+                throw new Error(stderr);
+            else
+                throw new Error(`Failed to print metrics`);
+        }
 
+        this.metrics = {};
         const prefix = 'metrics-' + this.config.problem + '/';
         for (let line of stdout.split('\n')) {
             let [key, value] = line.split('=');