[include-mapping] Print an error message in case the symbol index points to a non-existent page.

[VitaNuo]

Change implementation from using "print" to using "sys.stderr.write".

[kadircet]

thanks!

clang/tools/include-mapping/cppreference_parser.py

149

i think it's useful to mention the symbol name as well, maybe something like:

sys.stderr.write("Discarding information for '%s', page '%s' doesn't exist." % (symbol_name, path))

[VitaNuo]

Mention symbol name in the error message.

clang/tools/include-mapping/cppreference_parser.py

for symbol_name, symbol_page_path, variant in _ParseIndexPage(f.read()):
       variants_for_symbol = variants_to_accept.get(
           (namespace or "") + symbol_name, ())
       if variant and variant not in variants_for_symbol:
         continue
       path = os.path.join(root_dir, symbol_page_path)
       if os.path.isfile(path):
         results.append((symbol_name,
                       pool.apply_async(_ReadSymbolPage, (path, symbol_name))))
+      else:
+        sys.stderr.write("Could not parse page: %s. Page does not exist.\n" % path)                
 
     # Build map from symbol name to a set of headers.
     symbol_headers = collections.defaultdict(set)
     for symbol_name, lazy_headers in results:
       symbol_headers[symbol_name].update(lazy_headers.get())
 
   symbols = []
   for name, headers in sorted(symbol_headers.items(), key=lambda t : t[0]):