libdpkg: Clarify reason for multiarch database inconcistencies

The most probable cause for these inconsistencies are upgrades from
unofficial dpkg versions, make that clear to avoid users blaming the
official version.

debian/changelog

@@ -44,6 +44,8 @@ dpkg (1.16.9) UNRELEASED; urgency=low
     conffile checks actually work on reinstallation. Closes: #684776
   * Avoid info database corruption and bogus accesses on unknown format
     values, by always reading the format file and validating it.
+  * Clarify that the most probable reason for multiarch database
+    inconcistencies is due to upgrades from unofficial dpkg versions.
 
   [ Updated programs translations ]
   * Czech (Miroslav Kure).

lib/dpkg/parse.c

@@ -342,11 +342,13 @@ parse_find_set_slot(struct parsedb_state *ps,
     parse_count_pkg_instance(&count, pkg, &pkg->installed);
 
   if (count.single > 1)
-    parse_error(ps, _("multiple non-coinstallable package instances present"));
+    parse_error(ps, _("multiple non-coinstallable package instances present; "
+                      "most probably due to an upgrade from an unofficial dpkg"));
 
   if (count.single > 0 && count.multi > 0)
     parse_error(ps, _("mixed non-coinstallable and coinstallable package "
-                      "instances present"));
+                      "instances present; most probably due to an upgrade "
+                      "from an unofficial dpkg"));
 
   if (pkgset_installed_instances(set) != count.total)
     internerr("in-core pkgset '%s' with inconsistent number of instances",