Importer: Queue up log messages that happen before the importer prompt and show them all at once

Dear devs,

by your request cross-post from: https://github.com/beetbox/pyacoustid/issues/34
But: I suspect prints to stdout/stderr from pyacoustid, so this might not a beet issue. Anyway, I didn't take a look into the code yet.

I saw a lot of issues with this plugin. Now I read all of them, and nothing helped.

## versions
```
$ beet version
beets version 1.4.3
Python version 2.7.12
plugins: chroma, fetchart
```

```
$ sudo -H pip install -U pyacoustid
Requirement already up-to-date: pyacoustid in /usr/local/lib/python2.7/dist-packages
Requirement already up-to-date: audioread in /usr/local/lib/python2.7/dist-packages (from pyacoustid)
Requirement already up-to-date: requests in /usr/local/lib/python2.7/dist-packages (from pyacoustid)
```

```
$ which fpcalc
/usr/bin/fpcalc
```

## description

`$ beets import -s _unsortiert`

All it gives:
`chroma: fingerprinting of '/media/usb-hdd/Musik/_unsortiert/track.mp3' failed: audio could not be decoded`

No, the verbose options don't add any info. But oddly, I see the fingerprint which was successfully read.

Now when I execute fpcalc:
```
$ fpcalc Music/2-01\ track.mp3
[mp3 @ 0x1cad4a0] Header missing
WARNING: error decoding audio
```
But afterwards it does give me the correct fingerprint. So it _**seems**_ to work.

The bad thing is that I can't use beet anymore, because my screen gets spammed with error messages, and I have to scroll WAY up to see what beets wants from me (Skip? Not? Select?).

I think this needs fixing -- please supress these messages or make them print synchronously at last.
Also, how do I see if my files were identified by tags or by acousid? I haven't seen a hint yet.

## actual behaviour
My screen gets spammed with fingerprinting failed -- audio could not be decoded.
But it SEEMS it works, but there is no way to know.

## expected behaviour
Screen not getting spammed would be a start ;-)
Tell the user, if it actually failed, or if the corrupt header was silently ignored. Or don't tell him if it could be ignored.
Ask the user what to do if the file could not be read.
Tell the user in a better way that the current track was identified by acoustid.