Fix movie width and frame_rate returning nil (#14357)

* Fix movie width and frame_rate returning nil

* Add StreamValidationError and raise

* Fix code style

app/lib/exceptions.rb

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