Patch: Add Netpbm messages to verbose output

[Masamichi HOSODA]

I've uploaded the patch
that can add Netpbm messages to verbose output.

http://code.google.com/p/lilypond/issues/detail?id=4420
https://codereview.appspot.com/240950043

Until now, the Netpbm message didn't appear in the verbose output.
Therefore, it was difficult to understand Netpbm error
like following mail's issue.

This patch makes easy to analyse it.

From: Masamichi HOSODA <address@hidden>
Subject Re: cygwin64 - building 2.18.2 doc fails
Date Mon, 13 Apr 2015 23:29:38 +0900 (JST)

> lilypond-2.18.2's ``input/regression/midi/GNUmakefile'' (without my patch)
> uses TMP as a temporary variable.
> Therefore, it is set like following value the GNUmakefile.
> 
> TMP=key-initial.ly key-option-all-staves.ly key-option.ly lyrics-addlyrics.ly 
> partcombine.ly quantize-duration-2.ly quantize-duration.ly quantize-start.ly 
> rest-dynamic.ly rest.ly staff-map-instrument.ly staff-map-voice.ly voice-2.ly 
> voice-4.ly voice-5.ly
> 
> Then, pnmtopng fails with the error such as the following.
> 
> pnmtopng: Unable to create temporary file according to name pattern 
> 'key-initial.ly key-option-all-staves.ly key-option.ly lyrics-addlyrics.ly 
> partcombine.ly quantize-duration-2.ly quantize-duration.ly quantize-start.ly 
> rest-dynamic.ly rest.ly staff-map-instrument.ly staff-map-voice.ly voice-2.ly 
> voice-4.ly voice-5.ly/pnmtopng_XXXXXX'.  mkstemp() failed with errno 2 (No 
> such file or directory)