search._xmatch: handle aux_get KeyError (525718)

Since commit 55c8c8bc7a781e3f71ce92922eea64ad4cafce3c, emerge --search
can raise an unhandled KeyError if a pkg_desc_index contains a stale
package. Handle it with the same error message used for other aux_get
failures.

Fixes: 55c8c8bc7a78 ("Add emerge --search-index option.")
X-Gentoo-Bug: 525718
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=525718