fetchart and thumbnails: When `remove_art_file` is enabled, image is deleted before thumbnail hook runs

```
Ubuntu 14.04 64 bits
beets git version 1.3.16
Nautilus 3.10.1
```

```
directory: "/home/simon/M\xFAsica/"
library: /home/simon/.beets/musiclibrary.blb
threaded: no

plugins: convert fetchart info duplicates badfiles zero replaygain embedart lastgenre scrub chroma discogs thumbnails

fetchart:
auto: yes
maxwidth: 300

embedart:
auto: yes
maxwidth: 300
remove_art_file: yes

match:
strong_rec_thresh: 0.30
max_rec:
missing_tracks: strong

import:
art: yes
languages: es en

paths:
default: $albumartist/$album/$track - $title
singleton: $artist - $title
comp: Varios/$album/$track - $title

replaygain:
backend: bs1770gain
overwrite: yes

lastgenre:
canonical: yes

convert:
auto: yes
tmpdir: /tmp
command: ffmpeg -i $source -y -codec:a libmp3lame -vn -qscale:a 1 $dest
extension: mp3
quiet: yes
never_convert_lossy_files: yes

duplicates:
path: yes

thumbnails:
auto: yes
force: yes

scrub:
auto: yes

zero:
fields: comments
update_database: true
```

The image (`cover.jpg`) is in folder with mp3 files:

```
$ ls Música/Lana\ Del\ Rey/Honeymoon/
01 - Honeymoon.mp3 06 - Freak.mp3 11 - The Blackest Day.mp3
02 - Music to Watch Boys To.mp3 07 - Art Deco.mp3 12 - 24.mp3
03 - Terrence Loves You.mp3 08 - Burnt Norton (interlude).mp3 13 - Swan Song.mp3
04 - God Knows I Tried.mp3 09 - Religion.mp3 14 - Don’t Let Me Be Misunderstood.mp3
05 - High by the Beach.mp3 10 - Salvatore.mp3 cover.jpg
```

...but `beet thumbnails` doesn't find it:

```
$ beet -vv thumbnails 'lana del rey'
user configuration: /home/simon/.config/beets/config.yaml
data directory: /home/simon/.config/beets
plugin paths:
Sending event: pluginload
artresizer: method is (2, (6, 7, 7))
thumbnails: using IM to write metadata
thumbnails: using GIO to compute URIs
library database: /home/simon/.beets/musiclibrary.blb
library directory: /home/simon/Música
Sending event: library_opened
thumbnails: using IM to write metadata
thumbnails: using GIO to compute URIs
thumbnails: generating thumbnail for Lana Del Rey - Honeymoon
thumbnails: album Lana Del Rey - Honeymoon has no art
Sending event: cli_exit
```