Change handling of unknown extentions in validtor. (#1951)

This commit will change the message for unknown extensions from an error
to a warning.

Code was added to limit the number of warning messages so that consummer
of the messages are not overwhelmed.  This is standard practice in
compilers.

Many other issues were found at while looking into this. They have been
documented in #1950.

Fixes http://crbug.com/875547.