mesh: Fix sending messages when not provisioned
mesh: Fix sending messages when not provisioned

The message sending APIs should fail cleanly if we are not yet
provisioned.

IMGJulian/incubator-mynewt-core,IMGJulian/incubator-mynewt-core,mlaz/mynewt-core,andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,IMGJulian/incubator-mynewt-core,mlaz/mynewt-core,andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,mlaz/mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,IMGJulian/incubator-mynewt-core