Refactor main.go: improve flag handling and error validation

[betrayalove]

-- Refactored flag handling by separating validation logic into functions for better clarity and structure.
-- Added validation for flags based on the type of network (with or without fault proofs).
-- Improved error handling by centralizing error checks and using log.Crit for consistent error reporting.
-- Simplified the withdrawal process by moving the logic for proof and finalization into a dedicated function (processWithdrawal).
-- Enhanced readability and maintainability by dividing code into smaller, purpose-specific functions.

[cb-heimdall]

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