Update checking improvements (#38)

Still no auto retry for rate-limit. Instead, rate-limit errors are ignored and the unchecked Apps have to wait until the next cycle. Even this needs more testing before release.

lib/custom_errors.dart

@@ -0,0 +1,8 @@
+class RateLimitError {
+  late int remainingMinutes;
+  RateLimitError(this.remainingMinutes);
+
+  @override
+  String toString() =>
+      'Rate limit reached - try again in $remainingMinutes minutes';
+}