SWFmill did not always use non-zero exit status on failure

[nachanon]

From bug #33, I have used originally make to build an SWF file, which invoked SWFmill (simple mode).
But when SWFmill failed (.wav codec error), it simply exited with return code 0 (instead of nonzero code).

$ swfmill simple imaadpcm4play-stereo.xml imaadpcm4play-stereo.lib.swf
Error: Format code in WAVE file is not PCM (17)
Error: fmt_ subchunk size != 16. Is format code PCM?
WARNING: this file is not a valid WAVE file: siren-stereo-imaadpcm4.wav
xmlXPathCompiledEval: evaluation failed
runtime error: element apply-templates
Failed to evaluate the 'select' expression.
$ echo $?
0
$ 

This makes the use of SWFmill in scripts and makefile unreliable.

Test file could be downloaded from http://www.mediafire.com/?09db3whqa5z0kzq
(Run command above inside the extracted folder)

Note: SWFmill seems to correctly exited with non-zero code if it failed due to XML error.

SWFmill: 0.3.3 (source)
System: Debian GNU/Linux 7.0 Wheezy i386