Refactor expect to Result<> mapping

[Empty2k12]

Description

Refactors a possible panic to use graceful error handling

Checklist

• [x] Formatted code using cargo fmt --all
• [ ] Linted code using clippy
  • [ ] with reqwest feature: cargo clippy --manifest-path influxdb/Cargo.toml --all-targets --no-default-features --features serde,derive,reqwest-client-rustls -- -D warnings
  • [ ] with surf feature: cargo clippy --manifest-path influxdb/Cargo.toml --all-targets --no-default-features --features serde,derive,hyper-client -- -D warnings
• [x] Updated README.md using cargo doc2readme -p influxdb --expand-macros
• [x] Reviewed the diff. Did you leave any print statements or unnecessary comments?
• [ ] Any unfinished work that warrants a separate issue captured in an issue with a TODO code comment