[testapp]: Print stacktrace when write fails
[testapp]: Print stacktrace when write fails

We do make a core dump by calling abort(), but this printout
gives you the callstack right away rather than having to open
the debugger.

Change-Id: Ibfc4276b0d8962b4357879b9fbdc5a4c6852532e
Reviewed-on: http://review.couchbase.org/60195
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Daniel Owen <b3fc0e1a51f252986e60be7da121b74d3ac6845f@couchbase.com>

daverigby/memcached,owendCB/memcached,owendCB/memcached,couchbase/memcached,owendCB/memcached,owendCB/memcached,daverigby/memcached,daverigby/memcached,daverigby/memcached,daverigby/kv_engine,couchbase/memcached,couchbase/memcached,daverigby/kv_engine,daverigby/kv_engine,couchbase/memcached,daverigby/kv_engine