Uncommon logger levels in Friendica (#5453)

* "normal" is an uncommon logger level:
- changed LOGGER_NORMAL -> LOGGER_INFO
- added LOGGER_WARNING (a common logger level)

* Used constants instead of values (MrPetovan)

src/Database/DBStructure.php

@@ -61,7 +61,7 @@ class DBStructure
 
 		// No valid result?
 		if (!DBM::is_result($adminlist)) {
-			logger(sprintf('Cannot notify administrators about update_id=%d, error_message=%s', $update_id, $error_message), LOGGER_NORMAL);
+			logger(sprintf('Cannot notify administrators about update_id=%d, error_message=%s', $update_id, $error_message), LOGGER_INFO);
 
 			// Don't continue
 			return;