implement the Unwrap method for custom error types

[favonia]

Description

Since the introduction of error wrapping in Go 1.13, more and more Go errors implement the Unwrap method to work with the new error checking paradigm using errors.{Is,As}. This PR is to implement such an Unwrap method for custom error types defined in this library that wrap around a private Error. By implementing Unwrap that returns the private Error, these custom errors will be automatically seen by the Go standard library as wrappers of the private Error. Here's some sample code enabled by this PR:

var cloudflareErr cloudflare.Error
if errors.As(err, &cloudflareErr) {
	fmt.Printf("HTTP status code: %d", cloudflareErr.StatusCode)
}

This is extremely useful when trying to distinguish "errors from the Cloudflare API itself" and "errors due to other reasons (e.g., network errors)". Also, for reasons, only cloudflare.Error implements the ErrorMessageContains method. In sum, the Unwrap function gives users a clean interface to retrieve the private Error that works very well with the current Go standard library.

Has your change been tested?

Not yet. I'm worried a test involving errors.As would not run on platforms using older Go compilers.

Types of changes

What sort of change does your code introduce/modify?

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.

[github-actions]

changelog detected ✅

[favonia]

I now see that the 2.x library already has all the goodies. Perhaps this PR can be seen as an attempt to make 0.x more usable before completely abandoning it.

[jacobbednarz]

when it comes to the Cloudflare errors, you really don't want to be using Unwrap as it is here since it may contain multiple errors and you need to handle that in something like Error() to get the nicer formatting/handling (which could always be ported over but isn't what we're doing here). i.e. https://github.com/cloudflare/terraform-provider-cloudflare/blob/b33577b3af8207bc41b5ab4ae3ffe66f53ee8ff7/internal/sdkv2provider/resource_cloudflare_access_keys_configuration.go#L57-L64

we can add this here, however, going forward, we limiting what we do in 0.x as it detracts effort from the automatically generated versions.

[github-actions]

This functionality has been released in v0.103.0.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!