Fix govet issues

Change-Id: I4fb957ed10270ae3c17b458fcc7d7e11cea61417
diff --git a/package_ctx.go b/package_ctx.go
index cedee04..8e27150 100644
--- a/package_ctx.go
+++ b/package_ctx.go
@@ -96,7 +96,7 @@
 	checkCalledFromInit()
 
 	if _, present := packageContexts[pkgPath]; present {
-		panic(fmt.Errorf("package %q already has a package context"))
+		panic(fmt.Errorf("package %q already has a package context", pkgPath))
 	}
 
 	pkgName := pkgPathToName(pkgPath)
diff --git a/unpack_test.go b/unpack_test.go
index b33ae79..3efd447 100644
--- a/unpack_test.go
+++ b/unpack_test.go
@@ -224,7 +224,7 @@
 		[]error{
 			&Error{
 				Err: fmt.Errorf("filtered field nested.foo cannot be set in a Blueprint file"),
-				Pos: scanner.Position{"", 27, 4, 8},
+				Pos: mkpos(27, 4, 8),
 			},
 		},
 	},
@@ -399,3 +399,11 @@
 		}
 	}
 }
+
+func mkpos(offset, line, column int) scanner.Position {
+	return scanner.Position{
+		Offset: offset,
+		Line:   line,
+		Column: column,
+	}
+}