images/fastjpeg: use jpeg.Decode when djpeg fails.

Return error of type ErrDjpegFailed when djpeg or parsing the PNM returned
fails.
Attempt to decode image again with standard library.

Change-Id: I2a0cb7b52885732b7cbbffb8e34993d232781bc0
Merge "images/fastjpeg: use jpeg.Decode when djpeg fails."
Change strutil.StringFromBytes to depend on Go 1.3.

Change-Id: I83fd1369a199dba7ef41227df4d0b95d9a16aabb