beet update: Do not stop when files (including album art) cannot be moved

### Problem

Running `beet up` fails for me with messages like
~~~
moving album art /srv/music/beets/ASP/Aus der Tiefe_ Der schwarze Schmetterling, Teil IV/cover.jpg to /srv/music/beets/ASP/Aus der Tiefe_ Der schwarze Schmetterling, Teil IV [5622]/cover.1.
jpg
Traceback (most recent call last):
File "/srv/home/azrael/dev/beets/upstream/beets/util/__init__.py", line 470, in move
shutil.copyfile(path, dest)
File "/usr/lib64/python2.7/shutil.py", line 82, in copyfile
with open(src, 'rb') as fsrc:
IOError: [Errno 2] No such file or directory: '/srv/music/beets/ASP/Aus der Tiefe_ Der schwarze Schmetterling, Teil IV/cover.jpg'
~~~

### Setup

* OS: Fedora 23
* Python version: 2.7.11
* beets version: v1.4.3-71-g0a13240 (git HEAD)
* Turning off plugins made problem go away (yes/no):

My configuration (output of `beet config`) is:

```yaml
importfeeds:
relative_to: /srv/music
dir: /srv/music/lib/beets_importfeeds
formats: m3u
absolute_path: no
m3u_name: imported.m3u

paths:
singleton: $albumartist/$album%aunique{}/$track $title
comp: $albumartist/$album%aunique{}/$track $title
play:
command: bash -c 'cat $0 | mpc add'
relative_to: /srv/music
use_folders: no
raw: no
warning_threshold: 100
mpdstats:
host: localhost
port: 6600
music_directory: /srv/music/
rating: no
chroma:
auto: no
convert: (...)
embedart:
auto: no
compare_threshold: 0
ifempty: no
remove_art_file: no
maxwidth: 0
library: ~/.share/beets/library.blb

plugins: acousticbrainz embedart fetchart fromfilename info missing duplicates chroma info convert importfeeds play
directory: /srv/music/beets

import:
move: yes
log: ~/.share/beets/import.log
fetchart:
auto: yes
minwidth: 0
sources:
- filesystem
- coverart
- itunes
- amazon
- albumart
google_engine: 001442825323518660753:hrh5ch1gjzm
enforce_ratio: no
cautious: no
maxwidth: 0
store_source: no
google_key: REDACTED
fanarttv_key: REDACTED
cover_names:
- cover
- front
- art
- album
- folder
duplicates:
count: no
full: no
format: ''
keys: []
move: ''
tag: ''
path: no
copy: ''
tiebreak: {}
album: no
strict: no
checksum: ''
merge: no
delete: no
missing:
count: no
total: no
acousticbrainz:
auto: yes
force: no

```