improve error message

cmdline/apt-get.cc

@@ -322,9 +322,12 @@ pkgSrcRecords::Parser *FindSrc(const char *Name,pkgRecords &Recs,
 
       if (Src == "" && ArchTag != "")
       {
-          _error->Error(_("Can not find a package '%s' with version '%s' and "
-                          "release '%s'"), Pkg.FullName().c_str(), 
-                                           VerTag.c_str(), RelTag.c_str());
+         if (VerTag != "")
+            _error->Error(_("Can not find a package '%s' with version '%s'"),
+                          Pkg.FullName().c_str(), VerTag.c_str());
+         if (RelTag != "")
+            _error->Error(_("Can not find a package '%s' with release '%s'"),
+                          Pkg.FullName().c_str(), RelTag.c_str());
          Src = Name;
          return 0;
       }

test/integration/test-apt-get-source-arch

@@ -61,10 +61,10 @@ E: Unable to find a source package for foo:not-a-available-arch" aptget source -
 
 # error on unavailable version for arch
 testequal "$HEADER
-E: Can not find a package 'foo:amd64' with version '2.0' and release ''
+E: Can not find a package 'foo:amd64' with version '2.0'
 E: Unable to find a source package for foo:amd64=2.0" aptget source -q --print-uris foo:amd64=2.0
 
 # error on unavailable release for arch 
 testequal "$HEADER
-E: Can not find a package 'foo:amd64' with version '' and release 'oldstable'
+E: Can not find a package 'foo:amd64' with release 'oldstable'
 E: Unable to find a source package for foo:amd64/oldstable" aptget source -q --print-uris foo:amd64/oldstable