Detect leading spaces in config files

Because those will cause PHP to output to stdout and will cause
unrelated error messages.

Signed-off-by: Vincent Petry <vincent@nextcloud.com>

lib/private/Config.php

@@ -231,6 +231,14 @@ class Config {
 
 			unset($CONFIG);
 			include $file;
+			if (!defined('PHPUNIT_RUN') && headers_sent()) {
+				// syntax issues in the config file like leading spaces causing PHP to send output
+				$errorMessage = sprintf('Config file has leading content, please remove everything before "<?php" in %s', basename($file));
+				if (!defined('OC_CONSOLE')) {
+					print(\OCP\Util::sanitizeHTML($errorMessage));
+				}
+				throw new \Exception($errorMessage);
+			}
 			if (isset($CONFIG) && is_array($CONFIG)) {
 				$this->cache = array_merge($this->cache, $CONFIG);
 			}