FIX: Fatal error on create loan schedule

[lippoliv]

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FIX|Fix #30655

When handling errors of \LoanSchedule::create the view broke with a fatal php error.

1. Accessing old variable
2. Setting lines property to NULL instead of an empty array, as expected later

I fixed that and I set a more speaking, but still hardcoded error message in this case (it was "step1"). This probably should be discussed.

[lippoliv]

fyi: I forgot my local git config, so my commits are unverified. Let me know if this is a problem, then I create a new PR with "fully" verified commits ;)