builder: Print non-existent source files names in error message

Now error message specifies which source file could not be found
while building a project.
diff --git a/newt/builder/build.go b/newt/builder/build.go
index 5c0f323..50a1002 100644
--- a/newt/builder/build.go
+++ b/newt/builder/build.go
@@ -364,7 +364,6 @@
 	}
 
 	for _, filename := range bpkg.SourceFiles {
-		var dir string
 		repo, path, err := newtutil.ParsePackageString(filename)
 
 		if err != nil {
@@ -379,8 +378,8 @@
 
 		if util.NodeNotExist(filename) {
 			return nil, util.NewNewtError(fmt.Sprintf(
-				"Specified source directory %s, does not exist.",
-				dir))
+				"Specified source file %s, does not exist.",
+				filename))
 		}
 
 		entry, err := c.CollectSingleEntry(filename)