Fix invalid date searches returning 503 (#31526)

lib/exceptions.rb

@@ -8,6 +8,7 @@ module Mastodon
   class LengthValidationError < ValidationError; end
   class DimensionsValidationError < ValidationError; end
   class StreamValidationError < ValidationError; end
+  class FilterValidationError < ValidationError; end
   class RaceConditionError < Error; end
   class RateLimitExceededError < Error; end
   class SyntaxError < Error; end