Add more detail for errors where the problem is that TEMPDIR is filled up

communicator/ssh/communicator.go

@@ -804,7 +804,10 @@ func scpUploadFile(dst string, src io.Reader, w io.Writer, r *bufio.Reader, fi *
 
 		log.Println("[DEBUG] Copying input data into temporary file so we can read the length")
 		if _, err := io.Copy(tf, src); err != nil {
-			return err
+			return fmt.Errorf("Error copying input data into local temporary "+
+				"file. Check that TEMPDIR has enough space. Please see "+
+				"https://www.packer.io/docs/other/environment-variables.html#tmpdir"+
+				"for more info. Error: %s", err)
 		}
 
 		// Sync the file so that the contents are definitely on disk, then