[t:2449] Export test function (override loader's fwrite) via env. Fix bug in brtloader's handling of error from overridden fwrite().
Fix bug in ydb's handling of ephemeral loader when unable to write to disk.
Improve loader-cleanup-test.

git-svn-id: file:///svn/toku/tokudb@19825 c7de825b-a66e-492c-adef-691d508d4ae1