Print unparsable file in pkg-map

As you can see in the CI failures for
Ib11b9df84b593ab25232729a570c812f1b4b8774, you can not see what file
is causing the problems when the parser fails.  Add a print, but raise
the error as it contains the cause.

Change-Id: I127ff7d57d2d898969195464c6e774d496e872e6

elements/pkg-map/bin/pkg-map

@@ -74,7 +74,11 @@ def main():
         sys.exit(2)
 
     with open(map_file) as fd:
+        try:
             package_names = json.loads(fd.read())
+        except ValueError:
+            eprint('Unable to parse %s' % map_file)
+            raise
 
     # Parse mapping data in this form using distro/family/default
     # Most specific takes priority (distro is most specific).