forbid `unwrap` and `expect`

### Problem

there are some places that panics

### Proposed Solution

1. setup linter with forbidding [expect_used](https://rust-lang.github.io/rust-clippy/master/index.html#/expect_used) and [unwrap_used](https://rust-lang.github.io/rust-clippy/master/index.html#unwrap_used)
2. handle each `None` value and `Err` properly with fallbacks/failovers/retries/error propagation/etc

related to #350 