fix(multicall): impl Error for Failure + clear returns Empty builder.

[yash-atreya]

Motivation

1. We can't use ? on a multicall Result<(), Failure> since Failure doesn't implement StdError.
2. clear should return an Empty multicall builder otherwise it's of no use.

Solution

1. Use thiserror::Error for Failure
2. Return Empty multicall builder on multicall.clear()

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes