Ignore transient Tapicero errors when creating a db

tapicero runs into different couch errors from time to time, like this one:

C Dec 9 12:22:42 urchin tapicero[921]: Creating database user-032aa439429a6e1001f0dbfe40ed541e failed (trying again soon): RestClient::InternalServerError: 500 Internal Server Error
. Dec 9 12:22:47 urchin tapicero[921]: Prepared storage user-032aa439429a6e1001f0dbfe40ed541e

it's smart enough to try again, which works after a while (here: 5s)

we should ignore this particular error

(from redmine: created on 2014-12-09, closed on 2014-12-10, relates #6488 (closed))