[oil-language] Better error message when there's a trailing comma.

Although honestly I feel like I want the trailing comma?  It's harder to
parse, but more familiar to Python and Go programmers.

Have to think about it.

Related to #1135.

osh/cmd_parse.py

@@ -1178,7 +1178,10 @@ def _ParseForEachLoop(self, for_spid):
       if not ok or quoted:  # error: for $x
         p_die('Expected loop variable (a constant word)', word=self.cur_word)
       if not match.IsValidVarName(iter_name):  # error: for -
-        p_die("Invalid loop variable name", word=self.cur_word)
+        # TODO: consider commas?
+        if iter_name.endswith(','):
+          p_die('Remove comma; loop variables are separated by spaces', word=self.cur_word)
+        p_die('Invalid loop variable name %r', iter_name, word=self.cur_word)
 
       node.iter_names.append(iter_name)
       num_iter_names += 1

test/parse-errors.sh

@@ -1164,6 +1164,20 @@ oil_for() {
   done
   '
 
+  # NO COMMA!
+  _oil-parse-error '
+  for x, y in SPAM EGGS; do
+    echo $x
+  done
+  '
+
+  # Bad loop variable name
+  _oil-parse-error '
+  for x-y in SPAM EGGS; do
+    echo $x
+  done
+  '
+
   _oil-parse-error '
   for x y z in SPAM EGGS; do
     echo $x