refactor(errorsx): prepare for splitting the package (#476)

We will move the sane part of this package to i/netxlite/errorsx
and we will move the rest to i/e/legacy/errorsx.

What is the sane part? The sane part is error classifiers plus
the definition of ErrWrapper. The rest, including the rules
on how to decide whether an operation is major, are tricky and
we should consider them legacy and replace them with rules
that are more easy to understand and reason on.

Part of https://github.com/ooni/probe/issues/1591

internal/errorsx/resolver.go

@@ -23,19 +23,20 @@ var _ Resolver = &ErrorWrapperResolver{}
 func (r *ErrorWrapperResolver) LookupHost(ctx context.Context, hostname string) ([]string, error) {
 	addrs, err := r.Resolver.LookupHost(ctx, hostname)
 	err = SafeErrWrapperBuilder{
-		Classifier: classifyResolveFailure,
+		Classifier: ClassifyResolverError,
 		Error:      err,
 		Operation:  ResolveOperation,
 	}.MaybeBuild()
 	return addrs, err
 }
 
-// classifyResolveFailure is a classifier to translate DNS resolving errors to OONI error strings.
-func classifyResolveFailure(err error) string {
+// ClassifyResolverError maps an error occurred during a domain name
+// resolution to the corresponding OONI failure string.
+func ClassifyResolverError(err error) string {
 	if errors.Is(err, ErrDNSBogon) {
 		return FailureDNSBogonError // not in MK
 	}
-	return toFailureString(err)
+	return ClassifyGenericError(err)
 }
 
 type resolverNetworker interface {