Fix error handling of all the execSync usage

src/logcollector.js

@@ -22,6 +22,8 @@ function collectLogs(unitName, callback) {
     assert.strictEqual(typeof callback, 'function');
 
     var logs = safe.child_process.execSync('sudo ' + COLLECT_LOGS_CMD + ' ' + unitName, { encoding: 'utf8' });
+    if (!logs) return callback(safe.error);
+
     logs = logs + '\n\n=====================================\n\n';
 
     callback(null, logs);