Split expected and unexpected errors

[foresterre]

• errors are reported twice in the E in Result<T, E> as well as via the Reporter?
  

  cargo-msrv/src/sub_command/verify.rs

  Lines 95 to 99 in feef0f1

  	reporter.report_event(VerifyResult::incompatible(toolchain, error))?;
  	Err(CargoMSRVError::SubCommandVerify(Error::VerifyFailed(
  	VerifyFailed::from(rust_version),
  	)))

The returned errors from the Result values, are instead used for expected and unexpected internal errors, although errors may bubble up, and can be reported in the end.

I intended to separate expected and unexpected errors from each other by reporting expected errors (e.g. when a toolchain of a certain version fails to compile a project), from unexpected errors, but there is room for improvement here.

Originally posted in #753 (comment)

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• Expected errors should be reported to the user via the reporter
• expected errors can be converted from a Result Ei to a Ti where Ti From ; reporter.report(Ti)