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/media_file/image.rb

@@ -28,10 +28,22 @@ class MediaFile::Image < MediaFile
   end
 
   def is_corrupt?
+    error.present?
+  end
+
+  def error
+    image = Vips::Image.new_from_file(file.path, fail: true)
     image.stats
-    false
-  rescue Vips::Error
-    true
+    nil
+  rescue Vips::Error => e
+    # XXX Vips has a single global error buffer that is shared between threads and that isn't cleared between operations.
+    # We can't reliably use `e.message` here because it may pick up errors from other threads, or from previous
+    # operations in the same thread.
+    "libvips error"
+  end
+
+  def metadata
+    super.merge({ "Vips:Error" => error }.compact_blank)
   end
 
   def duration
@@ -44,7 +56,7 @@ class MediaFile::Image < MediaFile
     when :gif, :webp
       n_pages
     when :png
-      metadata.fetch("PNG:AnimationFrames", 1)
+      exif_metadata.fetch("PNG:AnimationFrames", 1)
     when :avif
       video.frame_count
     else
@@ -156,12 +168,12 @@ class MediaFile::Image < MediaFile
 
   # @return [Vips::Image] the Vips image object for the file
   def image
-    Vips::Image.new_from_file(file.path, fail: strict).autorot
+    Vips::Image.new_from_file(file.path, fail: false).autorot
   end
 
   def video
     FFmpeg.new(file)
   end
 
-  memoize :image, :video, :dimensions, :is_corrupt?, :is_animated_gif?, :is_animated_png?
+  memoize :image, :video, :preview_frame, :dimensions, :error, :metadata, :is_corrupt?, :is_animated_gif?, :is_animated_png?
 end