gst/gsterror.c: Add another error string used in a few existing plugins.

gst/gsterror.c: Add another error string used in a few existing plugins.

Original commit message from CVS:
* gst/gsterror.c: (_gst_stream_errors_init):
Add another error string used in a few existing plugins.

* gst/gstplugin.c:
* gst/gstpluginfeature.c: (gst_plugin_feature_load):
* tools/gst-inspect.c: (print_element_info):
When a feature disappears from a plugin (and the feature exists in
the cached registry file), things went horribly wrong. This isn't a
complete fix, we should actually be removing the 'missing' features
from the features list when we load the actual plugin. That's not
yet implemented.