AttributeError: TagReader instance has no attribute 'output_filename'

The follwing crash occurs, if searching for and installing a missing GStreamer plugin succeeds. Indeed, TagReader is a Decoder, not a Converter (which has attribute output_filename). Proposed fix is to use 'hasattr' for a safety-check and possibly apply a vfs_exists check prior to the unlink, too.

[...]

gstreamer.py:185:install_plugin_cb:AttributeError: TagReader instance has no attribute 'output_filename'

Traceback (most recent call last):
  File "/usr/lib/soundconverter/python/soundconverter/gstreamer.py", line 185, in install_plugin_cb
    vfs_unlink(self.output_filename)
AttributeError: TagReader instance has no attribute 'output_filename'

Local variables in innermost frame:
self: <soundconverter.gstreamer.TagReader instance at 0x1855128>
result: <enum GST_INSTALL_PLUGINS_SUCCESS of type GstInstallPluginsReturn>