Fix issue when non existing package is specified (#373)

This will make sure a proper error message is shown.

Author: ktf

alibuild_helpers/utilities.py

@@ -151,9 +151,9 @@ def __call__(self):
 
 def parseRecipe(reader):
   assert(reader.__call__)
+  err, spec, recipe = (None, None, None)
   try:
     d = reader()
-    err, spec, recipe = (None, None, None)
     header,recipe = d.split("---", 1)
     spec = yaml.safe_load(header)
     validateSpec(spec)