RFR(s): 8218543: ThreadsList handling during error reporting can crash

[Robbin Ehn]

Hi all, please review.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8218543
Changeset:
http://cr.openjdk.java.net/~rehn/8218543/v1/webrev/index.html

We avoid the assert and let the error reporting use the ThreadsList in an unsafe 
way. More discussion in issue.

Note that there was an get_first_error_tid(), but for some reason direct access 
to that field with friend was used. I change that to use the get method and 
renamed the field following private _ naming convention.

Passes t1-2

Thanks Robbin