Dpkg::Source::Package: Use GnuPG instead of gpg to refer to the software

The message is confusing as it refers to the command, but we might use
gpg2, also the message talks about the command not being installed when
in this context it would be expected a package. Instead use the software
name GnuPG.

debian/changelog

@@ -91,6 +91,8 @@ dpkg (1.18.5) UNRELEASED; urgency=medium
     - Do not parse entry multiple times in Dpkg::Changelog::Entry::Debian.
       Add new parse_header() and parse_trailer() methods, and deprecate
       check_header() and check_trailer() ones.
+    - Use “GnuPG” instead of “gpg” in error messages to refer to the software
+      in Dpkg::Source::Package.
   * Build system:
     - Fix building development documentation.
     - Remove unused UA_LIBS variable.

scripts/Dpkg/Source/Package.pm

@@ -445,9 +445,9 @@ sub check_signature {
         }
     } else {
         if ($self->{options}{require_valid_signature}) {
-            error(g_("could not verify signature on %s since gpg isn't installed"), $dsc);
+            error(g_('cannot verify signature on %s since GnuPG is not installed'), $dsc);
         } else {
-            warning(g_("could not verify signature on %s since gpg isn't installed"), $dsc);
+            warning(g_('cannot verify signature on %s since GnuPG is not installed'), $dsc);
         }
     }
 }