CBD-87: SFDSource.provide_design() exception handling
CBD-87: SFDSource.provide_design() exception handling

Also, using str() to avoid unicode in paths.
Also, PEP8 tweaks.

Change-Id: Id1edf81aa3a377e82fbfbea530fc8448ed07665b
Reviewed-on: http://review.couchbase.org/17498
Reviewed-by: Pavel Paulau <dd88eded64e90046a680e3a6c0828ceb8fe8a0e7@gmail.com>
Tested-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>

membase/membase-cli,membase/membase-cli,couchbase/couchbase-cli,couchbaselabs/couchbase-cli,couchbase/couchbase-cli,couchbaselabs/couchbase-cli,couchbaselabs/couchbase-cli,membase/membase-cli