media assets: track corrupted files in media metadata.

If a media asset is corrupt, include the error message from libvips or
ffmpeg in the "Vips:Error" or "FFmpeg:Error" fields in the media
metadata table.

Corrupt files can't be uploaded nowadays, but they could be in the past,
so we have some old corrupted files that we can't generate thumbnails
for. This lets us mark these files in the metadata so they're findable
with the tag search `exif:Vips:Error`.

Known bug: Vips has a single global error buffer that is shared between
threads and that isn't cleared between operations. So we can't reliably
get the actual error message because it may pick up errors from other
threads, or from previous operations in the same thread.

app/logical/ffmpeg.rb

@@ -41,8 +41,8 @@ class FFmpeg
     output = shell!("ffprobe -v quiet -print_format json -show_format -show_streams #{file.path.shellescape}")
     json = JSON.parse(output)
     json.with_indifferent_access
-  rescue Error
-    {}
+  rescue Error => e
+    { error: e.message.strip }.with_indifferent_access
   end
 
   def width
@@ -107,6 +107,16 @@ class FFmpeg
     audio_streams.present?
   end
 
+  # @return [Boolean] True if the video is unplayable.
+  def is_corrupt?
+    error.present?
+  end
+
+  # @return [String, nil] The error message if the video is unplayable, or nil if no error.
+  def error
+    metadata[:error] || playback_info[:error]
+  end
+
   # Decode the full video and return a hash containing the frame count, fps, and runtime.
   def playback_info
     output = shell!("ffmpeg -i #{file.path.shellescape} -f null /dev/null")
@@ -117,7 +127,7 @@ class FFmpeg
     info = status_line.scan(/\S+=\s*\S+/).map { |pair| pair.split(/=\s*/) }.to_h
     info.with_indifferent_access
   rescue Error => e
-    {}
+    { error: e.message.strip }.with_indifferent_access
   end
 
   def shell!(command)
@@ -127,5 +137,5 @@ class FFmpeg
     output
   end
 
-  memoize :metadata, :playback_info, :frame_count, :duration
+  memoize :metadata, :playback_info, :frame_count, :duration, :error
 end