Better error message when invalid amount

[diogeneshamilton]

Currently, if you send an invalid amount (like .01 which is really $.001), you get a response of "Invalid recipient (Please enter a valid phone, email, username, or Venmo user ID)", which is not helpful.

See here: http://stackoverflow.com/questions/25068307/ios-venmo-sdk-send-payment-issue-invalid-recipient-please-enter-a-valid-phone