#565 Conclude#follow raises Fbe::Error on second call

[VasilevNStas]

Fixes #565

Conclude#follow now raises Fbe::Error when called a second time, consistent with by and as in Fbe::Iterate.

Previously it silently overwrote @follows.

[VasilevNStas]

@yegor256 please review

[morphqdd]

Implements #565 — follow now raises instead of silently overwriting on a second call, matching by. I checked @follows is initialized to [] in the constructor, so the first call's @follows.empty? is safe. One gap inline.

[morphqdd]

This behavior change ships without a test. Since #565 is specifically about the second call, a test that calls follow twice and asserts the second raises Fbe::Error (and that a single call still works) would lock in the new contract.

[VasilevNStas]

Good catch. Added test_follow_raises_on_second_call — calls follow twice inside a conclude block and asserts the second raises Fbe::Error. Single-call path continues to work (verified by existing test_follow_multivalued).

[0crat]

@morphqdd Thanks for the review! You've earned +9 points for this: +18 as a basis; -5 for very few (1) comments; -4 for too few (1) hits-of-code. Your running score is +698; don't forget to check your Zerocracy account too).

[0crat]

@VasilevNStas Thanks for the contribution! You've earned +20 points for this: +24 as a basis; -4 for too few (22) hits-of-code. Please, keep them coming. Your running score is +2000; don't forget to check your Zerocracy account too).