NvpPluginException mixes err_msg and err_desc

In a few places err_msg was passed to NvpPluginException though that
expected err_desc. This patch makes NvpPluginException take err_msg
instead and updates err_desc to err_msg when calling NvpPluginException

Fixes bug 1098351
Change-Id: I10eca5f2b98ce8faa81ad7c7687902b6f08752c1