Remove thiserror dep to avoid indirect dep on syn

[bheylin]

Is your feature request related to a problem? Please describe.

I use inquire in an xtask style build system for a couple of projects.
I want the xtask compile time to be as short as possible and to that end I want to remove as many dependencies
from the Cargo.lock file as possible. The fact that inquire uses thiserror means everyone that uses inquire has to depends on syn which is a bottleneck in many project's build process.

Describe the solution you'd like
I would like to remove thiserror and implement the Error by hand.

Describe alternatives you've considered
onlyerror could be used instead of thiserror but given that inquire only
has one Error I consider both to be overkill.

Additional context
The readme of inquire lists the use of thiserror as a feature, in that it provides standardized error handling.
thiserror is designed to have no effect on the API of the crate and so it's the Rust std::error::Error that's providing
the standardized error handling. The goal of thiserror is to make defining errors quick and easy.