add more detail to errors

[jeremyandrews]

The goal of this PR is to make errors more rich, allowing application developers the option of what they want to display. In our examples, main() returns the errors so they are simply displayed.

• enriched GooseError::InvalidHost, for example:

  $ cargo run --example drupal_loadtest -- --host=foo
      Finished dev [unoptimized + debuginfo] target(s) in 0.08s
       Running `target/debug/examples/drupal_loadtest --host=foo`
  Error: InvalidHost { host: "foo", detail: Some("Failure parsing host specified with --host"), parse_error: RelativeUrlWithoutBase }
  

• enriched GooseError::FeatureNotEnabled, for example:

  $ cargo run --example drupal_loadtest -- --worker
      Finished dev [unoptimized + debuginfo] target(s) in 0.08s
       Running `target/debug/examples/drupal_loadtest --worker`
  Error: FeatureNotEnabled { feature: "gaggle", detail: Some("goose must be recompiled with `--features gaggle` to start in worker mode") }
  

• enriched GooseError::InvalidWaitTime, for example:

       Running `target/debug/examples/drupal_loadtest`
  Error: InvalidWaitTime { min_wait: 5, max_wait: 1, detail: Some("min_wait can not be larger than max_wait") }
  
  

• enriched GooseError::InvalidWeight, for example:

        Running `target/debug/examples/drupal_loadtest`
   Error: InvalidWeight { weight: 0, detail: Some("weight of 0 not allowed") }

• enriched GooseError::NoTaskSets

       Running `target/debug/examples/drupal_loadtest`
  Error: NoTaskSets { detail: Some("no task sets defined") }
  

• enriched GooseError::InvalidOption

  $ cargo run --example drupal_loadtest -- --host https://127.0.0.1/ --expect-workers=10
      Finished dev [unoptimized + debuginfo] target(s) in 0.08s
       Running `target/debug/examples/drupal_loadtest --host 'https://127.0.0.1/' --expect-workers=10`
  Error: InvalidOption { option: "--expect-workers", value: "10", detail: Some("--expect-workers is only available when running in manager mode") }
  

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@jeremyandrews This will create an infinite loop as per rust-lang/rust#74892.

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It only works right now as we use the Debug and not the Display output for errors when returning from main.

[jeremyandrews]

see #128