E-Invoice improvements

[teadur]

If there are connectivity issues with e_invoice provider user gets internal error, we should catch the exception and show meaningful error page to user.

#012HTTPClient::ConnectTimeoutError (execution expired):#12 app/models/registrar.rb:100:in issue_prepayment_invoice'#012 app/models/deposit.rb:36:in issue_prepayment_invoice'#12 app/controllers/registrar/deposits_controller.rb:11:in `create

Possibly start a background job what retries the sending or mark the invoice as not sent and retry them in bulk from cron.

Currently it's impossible to resend e_invoices if sending them fails at first try.
internetee/e_invoice#2

Not sure if possible but it would be more consistant if we could include invoice.description in the e-invoices.