feat: validate docstatus of selected invoices still saved/submitted

[HKuz]

Same changes to address #40 for the version-14 branch. With the switch to use frappe.get_value I realized I was able to collapse the for loop into a comprehension. (If it's worth it, I can quickly refactor the v13 code too).

Update: grammar nit fixed in the error message

[agritheory]

@HKuz Can you refactor this change into a separate method, like I did here:
#47

[HKuz]

All set - I caught a few small things during testing and can make the changes in v-13:

• the ref_number is None for some txns, which would throw an error in the line joining values for invalid_records
• invalid_records was overwriting itself if there were more than one cancelled transaction -> changed it to a += and added a <br> at the end to separate multiple transactions
• minor typo in error message 'The follow document(s)' -> 'The following document(s)'