scrub: Catch error when re-embedding illegal album art

### Problem

Running import on MP4 files in my collection is causing beets to crash with the error

ValueError: MP4 files only supports PNG and JPEG images

Running this command in verbose (`-vv`) mode:

```sh
$ beet -vv import -t
```

Led to this problem:

[log.txt](https://github.com/beetbox/beets/files/878632/log.txt)

Here's a link to the music files that trigger the bug (if relevant):

### Setup

* OS: Linux
* Python version: 2.7.13
* beets version: 1.4.3
* Turning off plugins made problem go away (yes/no): No. However, running ```beet clearart``` prior to importing resolves the issue.

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

```yaml
plugins: fetchart convert scrub web inline embedart
directory: /export/Music/Cleaned
library: /config/musiclibrary.db
art_filename: albumart
threaded: yes
original_date: no
per_disc_numbering: yes

convert:
auto: no
ffmpeg: /usr/bin/ffmpeg
opts: -ab 320k -ac 2 -ar 48000
max_bitrate: 320
threads: 1

paths:
default: $albumartist/$album%aunique{}/%if{$multidisc, $disc-}$track $title
singleton: Non-Album/$artist - $title
comp: Compilations/$album%aunique{}/%if{$multidisc, $disc-}$track $title
albumtype_soundtrack: Soundtracks/$album/%if{$multidisc, $disc-}$track $title

item_fields:
multidisc: 1 if disctotal > 1 else 0

import:
write: yes
copy: no
move: yes
resume: ask
incremental: yes
quiet_fallback: skip
timid: no
log: /config/beet.log

lastgenre:
auto: yes
source: album

embedart:
auto: no

fetchart:
auto: yes

replaygain:
auto: no

scrub:
auto: yes

replace:
'^\.': _
'[\x00-\x1f]': _
'[<>:"\?\*\|]': _
'[\xE8-\xEB]': e
'[\xEC-\xEF]': i
'[\xE2-\xE6]': a
'[\xF2-\xF6]': o
'[\xF8]': o
'\.$': _
'\s+$': ''

web:
host: 0.0.0.0
port: 8337
```

Start by reproducing the crash with `beet -vv import -t` on an affected MP4 and review the linked log. Compare behavior with the `scrub`, `embedart`, and `fetchart` plugins enabled, noting that `beet clearart` avoids the problem. Done means importing the affected files no longer crashes when album art is not PNG or JPEG.

Written by the indexing model from the issue text.