uploads: fix "ArgumentError: string contains null byte" error

Fix an error when trying to upload a file larger than the file size
limit. In this case we tried to dump the whole HTTP response into the
error message, which included the binary file itself, which caused this
exception because it contained null bytes.

app/models/media_asset.rb

@@ -39,7 +39,7 @@ class MediaAsset < ApplicationRecord
 
   validates :md5, uniqueness: { conditions: -> { where(status: [:processing, :active]) } }
   validates :file_ext, inclusion: { in: %w[jpg png gif mp4 webm swf zip], message: "Not an image or video" }
-  validates :file_size, numericality: { less_than_or_equal_to: Danbooru.config.max_file_size }
+  validates :file_size, numericality: { less_than_or_equal_to: Danbooru.config.max_file_size, message: ->(asset, _) { "too large (size: #{asset.file_size.to_formatted_s(:human_size)}; max size: #{Danbooru.config.max_file_size.to_formatted_s(:human_size)})" } }
   validates :duration, numericality: { less_than_or_equal_to: MAX_VIDEO_DURATION, message: "must be less than #{MAX_VIDEO_DURATION} seconds", allow_nil: true }, on: :create # XXX should allow admins to bypass
   validate :validate_resolution, on: :create