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.rb

@@ -10,7 +10,7 @@ class MediaFile
   extend Memoist
   include ActiveModel::Serializers::JSON
 
-  attr_accessor :file, :strict
+  attr_accessor :file
 
   # delegate all File methods to `file`.
   delegate *(File.instance_methods - MediaFile.instance_methods), to: :file
@@ -122,11 +122,8 @@ class MediaFile
   # Initialize a MediaFile from a regular File.
   #
   # @param file [File] The image file.
-  # @param strict [Boolean] If true, raise errors if the file is corrupt. If false,
-  #   try to process corrupt files without raising any errors.
-  def initialize(file, strict: true, **options)
+  def initialize(file, **options)
     @file = file
-    @strict = strict
   end
 
   # @return [Array<(Integer, Integer)>] the width and height of the file
@@ -164,7 +161,15 @@ class MediaFile
     file.size
   end
 
+  # @return [ExifTool::Metadata] The metadata for the file. Subclasses may override this to add
+  #   extra non-ExifTool metadata, such as error messages, Ugoira frame delays, or ffprobe metadata.
+  #   This metadata may be slower to calculate than the raw `exif_metadata`.
   def metadata
+    exif_metadata
+  end
+
+  # @return [ExifTool::Metadata] The metadata for the file, as returned by ExifTool.
+  def exif_metadata
     ExifTool.new(file).metadata
   end
 
@@ -207,9 +212,14 @@ class MediaFile
     file_ext == :swf
   end
 
-  # @return [Boolean] true if the file is corrupted in some way
+  # @return [Boolean] True if the file is too corrupted to read or generate thumbnails without error.
   def is_corrupt?
-    false
+    error.present?
+  end
+
+  # @return [String, nil] The error message when reading the file, or nil if there are no errors.
+  def error
+    nil
   end
 
   # @return [Boolean] true if the file is animated. Note that GIFs and PNGs may be animated.
@@ -278,7 +288,7 @@ class MediaFile
       height: height,
       file_size: file_size,
       file_ext: file_ext,
-      mime_type: mime_type,
+      mime_type: mime_type.to_s,
       md5: md5,
       is_corrupt?: is_corrupt?,
       is_supported?: is_supported?,
@@ -302,5 +312,5 @@ class MediaFile
     end
   end
 
-  memoize :file_ext, :file_size, :md5, :metadata, :mime_type
+  memoize :file_ext, :file_size, :md5, :mime_type, :exif_metadata
 end