#579 Validate variable substitution in Award text

[VasilevNStas]

Problem

When ${var} is used in award or bylaw text, an undefined variable silently produces nil or "**nil**" in the output, hiding configuration errors.

Solution

Both Bill#line and Bylaw#line now check that the variable exists via key? before substitution. If undefined, a descriptive Fbe::Error is raised with the variable name and the failing text.

Closes #579

[VasilevNStas]

@yegor256 plz review

[yegor256]

@VasilevNStas no test?

[VasilevNStas]

@yegor256

Tests added: test_bill_raises_on_undefined_var and test_bylaw_raises_on_undefined_var

plz review

[yegor256]

@VasilevNStas conflicts here

[VasilevNStas]

@yegor256
I`m fixed it
plz re-review