RFR(S) 6545422: NativeErrors.java uses wrong path name in exec

[Mattias Tobiasson]

Hi,
Could you please review this test fix.

Problems with the current version:
It sometimes fails to find the correct executable for "native2ascii".
It throws NullPointerExceptions from multiple locations without logs. Probably because it checks null values with "assert", which may not be enabled.

Main Changes:
1. Use common test library to run native2ascii.
2. More real checks for null.
3. More logging.

bug:
https://bugs.openjdk.java.net/browse/JDK-6545422

webrev:
http://cr.openjdk.java.net/~ykantser/6545422/webrev.00/

Mattias