Balanced: Fix exception for invalid email

On a purchase or authorize call, if an email having an invalid format
was specified, it was causing an exception:

can't convert nil into String
lib/active_merchant/billing/gateways/balanced.rb:368:in `+'
lib/active_merchant/billing/gateways/balanced.rb:368:in `http_request'
lib/active_merchant/billing/gateways/balanced.rb:359:in

Now we're returning a proper Response with a message from Balanced about
the email being invalid.

Close #737.