Printing without an invoice number now errors instead of increments
(grafted from 10f8a9f6e401b05643cccc275a9480707787eb6b)

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extra : source : 10f8a9f6e401b05643cccc275a9480707787eb6b
Merge pull request #18 from ehuelsmann/1.4

Printing without an invoice number now errors instead of increments