Better handling of log file failures (#9539)

Fix issues when APP_LOG is empty, unwritable , or pointed at a directory.
APP_LOG takes precedence over $config['log_file'] now.

DO NOT DELETE THIS TEXT

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config/app.php

@@ -120,9 +120,9 @@ return [
     |
     */
 
-    'log' => env('APP_LOG', 'logs/librenms.log'),  // log to the default file, until boot
+    'log' => env('APP_LOG'),  // log file to write to
 
-    'log_level' => env('APP_LOG_LEVEL', 'debug'),
+    'log_level' => env('APP_LOG_LEVEL', 'error'),
 
     /*
     |--------------------------------------------------------------------------