fix(bft): return error when committee loading fails

[yasinlex]

Problem

In bft/bft.go Start() function, errors from LoadCommittee() and LoadCommitteeData() were only logged as warnings:

b.ValidatorSet, err = b.Controller.LoadCommittee(...)
if err != nil {
    b.log.Warn(err.Error())  // Just a warning!
}

This allowed the BFT to continue with a potentially nil validator set, which would cause panics when:

• GetValidator() is called (line 242)
• SendToReplicas() is called (lines 270, 346, 446, etc.)
• SelectProposerFromCandidates() is called (line 291)

Fix

Changed from logging warnings to returning errors:

b.ValidatorSet, err = b.Controller.LoadCommittee(...)
if err != nil {
    b.log.Error(err.Error())
    return  // Stop BFT initialization
}

Impact

• Before: Committee loading failure → BFT continues with nil validator set → panic
• After: Committee loading failure → BFT stops initialization → error logged