netxlite: improve docs, tests, and code quality (#493)

* netxlite: improve docs, tests, and code quality

* better documentation

* more strict testing of dialer (especially make sure we
document the quirk in https://github.com/ooni/probe/issues/1779
and we have tests to guarantee we don't screw up here)

* introduce NewErrWrapper factory for creating errors so we
have confidence we are creating them correctly

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

internal/netxlite/errorsx/errwrapper.go

@@ -67,3 +67,28 @@ func (e *ErrWrapper) Unwrap() error {
 func (e *ErrWrapper) MarshalJSON() ([]byte, error) {
 	return json.Marshal(e.Failure)
 }
+
+// Classifier is the type of function that performs classification.
+type Classifier func(err error) string
+
+// NewErrWrapper creates a new ErrWrapper using the given
+// classifier, operation name, and underlying error.
+//
+// This function panics if classifier is nil, or operation
+// is the empty string or error is nil.
+func NewErrWrapper(c Classifier, op string, err error) *ErrWrapper {
+	if c == nil {
+		panic("nil classifier")
+	}
+	if op == "" {
+		panic("empty op")
+	}
+	if err == nil {
+		panic("nil err")
+	}
+	return &ErrWrapper{
+		Failure:    c(err),
+		Operation:  op,
+		WrappedErr: err,
+	}
+}