Parser / PreParser: Simplify error message arguments.

In some places, we pretended that there can be multiple arguments, though in
practice there was only one. In other places (most importantly, PreParser), we
only handled one argument. (This means that we were not able to produce a
multi-argument error inside a lazy function anyway.)

This CL makes it clear that there is ever only one argument.

R=ulan@chromium.org
BUG=

Review URL: https://codereview.chromium.org/273653002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21324 ce2b1a6d-e550-0410-aec6-3dcde31c8c00