Reduce use of unwrap in CLI

[fairingrey]

There's a lot of unwrap use in https://github.com/spruceid/didkit/blob/c5c422f2469c2c5cc2f6e6d8746e95b552fce3ed/cli/src/main.rs that to an end user is probably not very useful to reveal the entire output of a panic to. Wrrors like file not found, malformed JSON, bad identifiers... where an error response is expected, but not where in particular the error happens in code.

Perhaps we could use https://docs.rs/anyhow and categorize errors into an enum CliError with more helpful error messages.

[clehner]

#255 is changing main to return a Result (anyhow::Result). unwrap() and expect(...) can then be changed to ? or .context(...)?. Error::source chains will appear in the stdout on error rather than the panic message.