Update erc20.rs

[0xLayer-3]

Changelog: erc20_transaction Safety & Logic Fixes

1. Removed unwrap() from user_address.parse()

Before:

let caller = user_address.parse().unwrap();

After:

let caller: Address = user_address.parse()?;

Reason:
unwrap() would panic if the provided user_address string was invalid or malformed. Replaced it with typed parsing and the ? operator to return a proper Err through the existing Result type. This ensures the function fails gracefully instead of crashing at runtime.

2. Added proper error handling for invalid ERC transfer variant

Before:

ERCTransferVariant::NA => {
error!("ERC should have a variant");
}

After:

ERCTransferVariant::NA => {
error!("ERC should have a variant");
return Err("ERC should have a valid transfer variant".into());
}

Reason:
The previous behavior only logged an error but still returned Ok((new_source, new_destination)), which falsely signaled success and could propagate inconsistent or incomplete state. Now, the function correctly returns an error when an invalid variant is encountered, making it impossible for the pipeline to continue with undefined logic.

Summary

Fixed potential runtime panic safer and more predictable error propagation.

Improved logical integrity invalid ERC transfer variants now terminate early with an explicit error.

No behavior change for valid paths; existing event and handle logic remains untouched

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