Improve error messages during build.

build.py

@@ -261,6 +261,7 @@ class Gen_compressed(threading.Thread):
       for error in errors:
         print('FATAL ERROR')
         print(error['error'])
+        if error['file']:
           print('%s at line %d:' % (
               file_lookup(error['file']), error['lineno']))
           print(error['line'])
@@ -272,6 +273,7 @@ class Gen_compressed(threading.Thread):
         for warning in warnings:
           print('WARNING')
           print(warning['warning'])
+          if warning['file']:
             print('%s at line %d:' % (
                 file_lookup(warning['file']), warning['lineno']))
             print(warning['line'])