Warn if preferred transfer mechanisms are not available

[nawatts]

Currently, the client sends to the MR a list of the mechanisms available to it. The MR replies with only transfers that use one of those mechanisms. Instead, it should reply with all transfers for the URL (the client doesn't need to send mechanisms). The client should run only the transfers with mechanisms that it has available and warn if any higher preference transfers would use a mechanism that it doesn't have available.