Convert the exec output to string (#239)

exec outputs an array of lines, handleError() only accepts strings.
This causes a TypeError due to strict_types, and is very hard to debug.

Co-authored-by: Jeroen Bakker <jeroen.bakker@yoursurprise.com>

lib/Consumer/ForkCurl.php

@@ -41,6 +41,7 @@ public function flushBatch(array $messages): bool
             exec($cmd2, $output, $exit);
 
             if ($exit !== 0) {
+                $output = implode(PHP_EOL, $output);
                 $this->handleError($exit, $output);
                 return false;
             }
@@ -75,6 +76,7 @@ public function flushBatch(array $messages): bool
         exec($cmd, $output, $exit);
 
         if ($exit !== 0) {
+            $output = implode(PHP_EOL, $output);
             $this->handleError($exit, $output);
         }