feat: clearer error message for arithmetic underflow

[neutiyoo]

This pull request includes several changes to the FERC20 contract to improve the handling of balances and allowances. The most important changes include adding balance and allowance checks, and using unchecked arithmetic operations to optimize gas usage.

Improvements to balance and allowance handling:

• Added a check to ensure the sender's allowance is sufficient before transferring tokens in the transferFrom function. (contracts/fun/FERC20.sol)
• Added a check to ensure the sender's balance is sufficient before transferring tokens in the _transfer function. (contracts/fun/FERC20.sol)
• Added a check to ensure the user's balance is sufficient before burning tokens in the _burn and burnFrom functions. (contracts/fun/FERC20.sol)

Optimizations:

• Used unchecked arithmetic operations in the _transfer and _burn functions to save gas. (contracts/fun/FERC20.sol) [1] [2]