Email owner if invoice creation failed

[Sjors]

If invoice creation fails, e.g. because the lightning node crashed, it currently just sends an exception notification:

[Matreon Exception]  (Net::ReadTimeout) "Net::ReadTimeout"

A Net::ReadTimeout occurred in background at 2018-07-06 23:32:14 UTC :

Net::ReadTimeout
/app/vendor/ruby-2.4.2/lib/ruby/2.4.0/net/protocol.rb:176:in `rbuf_fill'
-------------------------------
Backtrace:
[...]
/app/app/models/invoice.rb:103:in `create_lightning_charge_invoice'
/app/vendor/bundle/ruby/2.4.0/gems/activesupport-5.1.6/lib/active_support/callbacks.rb:413:in `block in make_lambda'
[...]
/app/app/models/contribution.rb:41:in `create_or_update_invoice!'
/app/app/models/invoice.rb:38:in `block in generate!'
/app/vendor/bundle/ruby/2.4.0/gems/activerecord-5.1.6/lib/active_record/relation/delegation.rb:39:in `each'
/app/vendor/bundle/ruby/2.4.0/gems/activerecord-5.1.6/lib/active_record/relation/delegation.rb:39:in `each'
/app/app/models/invoice.rb:37:in `generate!'
/app/lib/tasks/invoices.rake:8:in `block (2 levels) in <top (required)>'
[...]

This is much too cryptic for such an important error. This exception should be caught instead and result in the owner receiving an email that invoice creation failed (and some info as to why).