do not check odb_mkstemp return value for errors

The odb_mkstemp function does not return an error; it dies on failure instead. But many of its callers compare the resulting descriptor against -1 and die themselves. Mostly this is just pointless, but it does raise a question when looking at the callers: if they show the results of the "template" buffer after a failure, what's in it? The answer is: it doesn't matter, because it cannot happen. So let's make that clear by removing the bogus error checks. In bitmap_writer_finish(), we can drop the error-handling code entirely. In the other two cases, it's shared with the open() in another code path; we can just move the error-check next to that open() call. And while we're at it, let's flesh out the function's docstring a bit to make the error behavior clear. Signed-off-by: Jeff King <peff@peff.net>