Try `getRequiredTotal` on more than one server

download currently does:

someParam <- firstRightM lookupServer (getRequiredTotal verifier) (Map.elems servers)

For mutables, getRequiredTotal actually goes and talks to a server (because the encoding parameters are stored in shares, not in the capability string). However, if there is an error talking to the server then the result is an IO exception instead of Nothing. Since download doesn't handle the exception it never tries a second server.