spatch: throw error instead of making incorrect transformations

Summary: See the task

Test Plan:
$ ~/pfff/spatch -f transfo_on_comma_varargs.spatch transfo_on_comma_varargs.php
now throw an error

(and
$ ~/pfff/spatch -f transfo_on_comma_varargs_ok.spatch transfo_on_comma_varargs.php

does the right thing)

Reviewers: dhoskins, pieter, julienv

Reviewed By: pieter

Differential Revision: https://phabricator.fb.com/D786406

Task ID: 2324670
spatch: better error message when parse error in spatch file (changen related)

Summary:
One side effect of using changen() in any_of_string is that error
message got more cryptic. Let's fix that.

Test Plan:
$ ~/pfff/spatch -f missing_paren.spatch missing_paren.php
Fatal error: exception Failure("could not parse: foo (
 ...
")

and not anymore Sys_error no such file or directory

Reviewers: zoel, pieter, dhoskins, julienv, amenghra

Reviewed By: pieter

Differential Revision: https://phabricator.fb.com/D787307

Task ID: 2312554