uploads: improve error messages.

Improve upload error messages when downloading an URL fails, or it isn't
an image or video file.

app/logical/danbooru/http.rb

@@ -147,7 +147,7 @@ module Danbooru
       def download_media(url, file: Tempfile.new("danbooru-download-", binmode: true))
         response = get(url)
 
-        raise DownloadError, "Downloading #{response.uri} failed with code #{response.status}" if response.status != 200
+        raise DownloadError, "#{url} failed with code #{response.status}" if response.status != 200
         raise FileTooLargeError, "File size too large (size: #{response.content_length.to_i.to_formatted_s(:human_size)}; max size: #{@max_size.to_formatted_s(:human_size)})" if @max_size && response.content_length.to_i > @max_size
 
         size = 0

app/logical/sources/strategies/base.rb

@@ -219,9 +219,7 @@ module Sources
 
       # Download the file at the given url, or at the main image url by default.
       def download_file!(download_url = image_url)
-        raise DownloadError, "Download failed: couldn't find download url for #{url}" if download_url.blank?
         response, file = http_downloader.download_media(download_url)
-        raise DownloadError, "Download failed: #{download_url} returned error #{response.status}" if response.status != 200
         file
       end