better error message in RPC#dispatch

before:

/Users/fleuria/code/coinbaz/vendor/cache/ruby-wallet-0907d2fec6fd/lib/bitcoin/rpc.rb:32:in `rescue in dispatch': undefined method `response' for #<Errno::ECONNREFUSED: Connection refused - connect(2)> (NoMethodError)
    from /Users/fleuria/code/coinbaz/vendor/cache/ruby-wallet-0907d2fec6fd/lib/bitcoin/rpc.rb:26:in `dispatch'
    from /Users/fleuria/code/coinbaz/vendor/cache/ruby-wallet-0907d2fec6fd/lib/bitcoin/api.rb:32:in `request'
    from /Users/fleuria/code/coinbaz/vendor/cache/ruby-wallet-0907d2fec6fd/lib/bitcoin/client.rb:131:in `listaccounts'
    from /Users/fleuria/code/coinbaz/vendor/cache/ruby-wallet-0907d2fec6fd/lib/wallet/accounts.rb:13:in `initialize'

after:

/Users/fleuria/code/coinbaz/vendor/cache/ruby-wallet-0907d2fec6fd/lib/bitcoin/rpc.rb:32:in `rescue in dispatch': Connection Refused (Bitcoin::Errors::RPCError)
    from /Users/fleuria/code/coinbaz/vendor/cache/ruby-wallet-0907d2fec6fd/lib/bitcoin/rpc.rb:26:in `dispatch'
    from /Users/fleuria/code/coinbaz/vendor/cache/ruby-wallet-0907d2fec6fd/lib/bitcoin/api.rb:32:in `request'
    from /Users/fleuria/code/coinbaz/vendor/cache/ruby-wallet-0907d2fec6fd/lib/bitcoin/client.rb:131:in `listaccounts'
    from /Users/fleuria/code/coinbaz/vendor/cache/ruby-wallet-0907d2fec6fd/lib/wallet/accounts.rb:13:in `initialize'
    from /Users/fleuria/code/coinbaz/vendor/cache/ruby-wallet-0907d2fec6fd/lib/wallet/wallet.rb:21:in `new'