metadata: record metadata for corrupt files.

Bug: if ExifTool exited with status 1 because it thought the file was
corrupt, then we didn't record any of the metadata, even though it was
able to read most of it. It turns out there are thousands of posts with
minorly corrupt metadata that ExifTool is still able to read, but will
complain about.

Fix: ignore the exit code of ExifTool and always save whatever metadata
ExifTool is able to return. It will return an `ExifTool:Error` tag in
the event of errors.

Note that there are some (many?) files that are considered corrupt by
ExifTool but not by Vips, and vice versa. Probably because ExifTool only
parses the metadata while Vips only parses the image data.

app/logical/exif_tool.rb

@@ -4,8 +4,6 @@ require "shellwords"
 class ExifTool
   extend Memoist
 
-  class Error < StandardError; end
-
   # @see https://exiftool.org/exiftool_pod.html#OPTIONS
   DEFAULT_OPTIONS = %q(
     -G1 -duplicates -unknown -struct --binary
@@ -26,17 +24,11 @@ class ExifTool
   # @param options [String] the options to pass to exiftool
   # @return [Hash] the file's metadata
   def metadata(options: DEFAULT_OPTIONS)
-    output = shell!("exiftool #{options} -json #{file.path.shellescape}")
+    output = %x(exiftool #{options} -json #{file.path.shellescape})
     json = JSON.parse(output).first
     json = json.except("SourceFile")
     json.with_indifferent_access
   end
 
-  def shell!(command)
-    output, status = Open3.capture2e(command)
-    raise Error, "#{command}` failed: #{output}" if !status.success?
-    output
-  end
-
   memoize :metadata
 end

test/unit/media_file_test.rb

@@ -190,6 +190,40 @@ class MediaFileTest < ActiveSupport::TestCase
     end
   end
 
+  context "a corrupt GIF" do
+    should "still read the metadata" do
+      @file = MediaFile.open("test/files/test-corrupt.gif")
+      @metadata = @file.metadata
+
+      assert_equal(true, @file.is_corrupt?)
+      assert_equal("File format error", @metadata["ExifTool:Error"])
+      assert_equal("89a", @metadata["GIF:GIFVersion"])
+      assert_equal(6, @metadata.count)
+    end
+  end
+
+  context "a corrupt PNG" do
+    should "still read the metadata" do
+      @file = MediaFile.open("test/files/test-corrupt.png")
+      @metadata = @file.metadata
+
+      assert_equal(true, @file.is_corrupt?)
+      assert_equal("Grayscale", @metadata["PNG:ColorType"])
+      assert_equal(6, @metadata.count)
+    end
+  end
+
+  context "a corrupt JPEG" do
+    should "still read the metadata" do
+      @file = MediaFile.open("test/files/test-corrupt.jpg")
+      @metadata = @file.metadata
+
+      assert_equal(true, @file.is_corrupt?)
+      assert_equal(1, @metadata["File:ColorComponents"])
+      assert_equal(7, @metadata.count)
+    end
+  end
+
   context "a greyscale image without an embedded color profile" do
     should "successfully generate a thumbnail" do
       @image = MediaFile.open("test/files/test-grey-no-profile.jpg")