ENGCOM-2371: Log when Magento is in maintenance mode #16840

 - Merge Pull Request #16840 from Ethan3600/magento2:hotfix/log-maintenance-mode
 - Merged commits:
   1. ef616ce
   2. ac57387
   3. 19c3f3a
   4. 8d57d04

lib/internal/Magento/Framework/App/Bootstrap.php

@@ -12,6 +12,7 @@
 use Magento\Framework\Autoload\Populator;
 use Magento\Framework\Config\File\ConfigFilePool;
 use Magento\Framework\Filesystem\DriverPool;
+use Psr\Log\LoggerInterface;
 
 /**
  * A bootstrap of Magento application
@@ -258,6 +259,7 @@ public function run(AppInterface $application)
                 \Magento\Framework\Profiler::stop('magento');
             } catch (\Exception $e) {
                 \Magento\Framework\Profiler::stop('magento');
+                $this->objectManager->get(LoggerInterface::class)->error($e->getMessage());
                 if (!$application->catchException($this, $e)) {
                     throw $e;
                 }
@@ -423,7 +425,7 @@ protected function terminate(\Exception $e)
                 if (!$this->objectManager) {
                     throw new \DomainException();
                 }
-                $this->objectManager->get(\Psr\Log\LoggerInterface::class)->critical($e);
+                $this->objectManager->get(LoggerInterface::class)->critical($e);
             } catch (\Exception $e) {
                 $message .= "Could not write error message to log. Please use developer mode to see the message.\n";
             }