osd: drop newlines from event descriptions

These produce extra newlines in the log.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
osd: better error message for request on pool that dne

If the request is sent when the pool didn't even exist, say so.  This
would have made #3734 a bit easier to track down.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>