Log database errors instead of showing them.

display_errors is off unless DISPLAY_ERRORS is set in .env.

.env.example

@@ -17,3 +17,7 @@ GOOGLE_ANALYTICS_ID=
 # Directory for uploaded images waiting to be saved. Keep it outside the
 # web root. The default is xxx-uploads in the system temporary directory.
 UPLOAD_DIR=
+
+# Show PHP errors in the page (1) or on stderr (stderr). Leave it empty in
+# production: errors always go to the PHP error log.
+DISPLAY_ERRORS=