dpkg-vendor: Use Dpkg::Vendor::get_vendor_dir instead of literal string

Print the correct path to the vendor directory, overridable either at
configure or run-time.

debian/changelog

@@ -85,6 +85,7 @@ dpkg (1.17.0) UNRELEASED; urgency=low
   * Fix Dpkg::Control::Hash set_options() to take a hash as argument instead
     of two scalars.
   * Add a new Dpkg::Vendor get_vendor_dir() function.
+  * Print correct path to vendor directory on error message in dpkg-vendor.
 
   [ Updated manpages translations ]
   * Fix incorrect translation of "fortify" in French manpage for dpkg-buildflags

scripts/dpkg-vendor.pl

@@ -24,7 +24,7 @@ use warnings;
 use Dpkg;
 use Dpkg::Gettext;
 use Dpkg::ErrorHandling;
-use Dpkg::Vendor qw(get_vendor_info get_current_vendor);
+use Dpkg::Vendor qw(get_vendor_dir get_vendor_info get_current_vendor);
 
 textdomain('dpkg-dev');
 
@@ -84,8 +84,8 @@ $vendor //= get_current_vendor();
 
 my $info = get_vendor_info($vendor);
 unless (defined($info)) {
-    error(_g("vendor %s doesn't exist in /etc/dpkg/origins/"),
-          $vendor || 'default');
+    error(_g("vendor %s doesn't exist in %s"), $vendor || 'default',
+          get_vendor_dir());
 }
 
 if ($action eq 'is') {