fix(netxlite): ensure HTTP errors are always wrapped (#584)

1. introduce implementations of HTTPTransport and HTTPClient
that apply an error wrapping policy using the constructor
for a generic top-level error wrapper

2. make sure we use the implementations in point 1 when we
are constructing HTTPTransport and HTTPClient

3. make sure we apply error wrapping using the constructor for
a generic top-level error wrapper when reading bodies

4. acknowledge that error wrapping would be broken if we do
not return the same classification _and_ operation when we wrap
an already wrapped error, so fix the to code to do that

5. acknowledge that the classifiers already deal with preserving
the error string and explain why this is a quirk and why we
cannot remove it right now and what needs to happen to safely
remove this quirk from the codebase

Closes https://github.com/ooni/probe/issues/1860

internal/netxlite/errwrapper.go

@@ -1,6 +1,9 @@
 package netxlite
 
-import "encoding/json"
+import (
+	"encoding/json"
+	"errors"
+)
 
 // ErrWrapper is our error wrapper for Go errors. The key objective of
 // this structure is to properly set Failure, which is also returned by
@@ -78,7 +81,19 @@ type Classifier func(err error) string
 //
 // This function panics if classifier is nil, or operation
 // is the empty string or error is nil.
+//
+// If the err argument has already been classified, the returned
+// error wrapper will use the same classification string and
+// failed operation of the original wrapped error.
 func NewErrWrapper(c Classifier, op string, err error) *ErrWrapper {
+	var wrapper *ErrWrapper
+	if errors.As(err, &wrapper) {
+		return &ErrWrapper{
+			Failure:    wrapper.Failure,
+			Operation:  wrapper.Operation,
+			WrappedErr: err,
+		}
+	}
 	if c == nil {
 		panic("nil classifier")
 	}
@@ -97,6 +112,10 @@ func NewErrWrapper(c Classifier, op string, err error) *ErrWrapper {
 
 // NewTopLevelGenericErrWrapper wraps an error occurring at top
 // level using ClassifyGenericError as classifier.
+//
+// If the err argument has already been classified, the returned
+// error wrapper will use the same classification string and
+// failed operation of the original error.
 func NewTopLevelGenericErrWrapper(err error) *ErrWrapper {
 	return NewErrWrapper(ClassifyGenericError, TopLevelOperation, err)
 }