Cleanup __conn_dhandle_get() error handling (if btree allocation failed,
dhandle wasn't freed, if an error happened after btree allocation, it
was never freed).

Fix the __conn_dhandle_get() comment by removing most of it.
update
Clarify where we're freeing the btree structure.
Merge pull request #669 from wiredtiger/__conn_dhandle_get-error-handling

 clean up conn_dhandle_get() error handling