RFR 8188226: jextract diagnostic generation should be improved

[Sundararajan Athijegannathan]

Please review: http://cr.openjdk.java.net/~sundar/8188226/webrev.00/

* stack traces printed only on "debug" [when jextract.debug property is set]
* default log level set to "warning" [user can change log level by -log 
option already]
* added resource for localizing messages.

PS. Piggybacking to add missing copyright messages in few source files.

Thanks,
-Sundar