src/plugin.c: Print a more verbose error message if lt_dlopen() fails.

src/plugin.c: Print a more verbose error message if lt_dlopen() fails.

Since the Debian package doesn't depend on the libraries used by the
collectd plugins, some plugins may fail to load. ltdl reports this with
the very confusing error message "file not found". Since the plugin is
in fact available, many users don't realize a dependency is missing and
assume collectd is looking in the wrong directory -- and they are hardly
to blame for this.

This commit introduces a lengthy error message which hopefully points
users into the right direction.