Manifest: Fix error msg for missing yaml package

Fix the error message print instruction in
tools/tfm_parse_manifest_list.py for missing yaml package, to be
Python 3 compatible.

Change-Id: Ib4d1454dd9154231c9f9f2e69cfa3cbb12036dda
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>
diff --git a/tools/tfm_parse_manifest_list.py b/tools/tfm_parse_manifest_list.py
index ea6d118..1ed8b81 100644
--- a/tools/tfm_parse_manifest_list.py
+++ b/tools/tfm_parse_manifest_list.py
@@ -12,7 +12,7 @@
 try:
     import yaml
 except ImportError as e:
-    print (e + " To install it, type:")
+    print (str(e) + " To install it, type:")
     print ("pip install PyYAML")
     exit(1)