tools/gn: Give a better informative message if user uses apostrophe.

tools/gn/tokenizer.cc

Index: tools/gn/tokenizer.cc
diff --git a/tools/gn/tokenizer.cc b/tools/gn/tokenizer.cc
index 402b5ecaeb03ac49e777caaedc0158abaaa043c2..397888811303e705c123ab9727f3adea0585235b 100644
--- a/tools/gn/tokenizer.cc
+++ b/tools/gn/tokenizer.cc
@@ -388,6 +388,8 @@ Err Tokenizer::GetErrorForInvalidToken(const Location& location) const {
       (input_[cur_ + 1] == '/' || input_[cur_ + 1] == '*')) {
     // Different types of comments.
     help = "Comments should start with # instead";
+  } else if (cur_char() == '\'') {
+    help = "I expected \" but got ' instead.";

[Daniel Bratell, 2015/03/23 09:05:22]

Since this might also be a spurious ' in the middle of nothing, maybe:

"Strings are delimited by \" characters, not apostrophes."

If it was intended as a string, it will be obvious enough. If it was not
intended as a string, it will be obvious that it was about to be parsed as one.

[tfarina, 2015/03/23 10:40:51]

Done.

   } else {
     help = "I have no idea what this is.";
   }