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/iox.go

@@ -27,9 +27,9 @@ func ReadAllContext(ctx context.Context, r io.Reader) ([]byte, error) {
 	case data := <-datach:
 		return data, nil
 	case <-ctx.Done():
-		return nil, ctx.Err()
+		return nil, NewTopLevelGenericErrWrapper(ctx.Err())
 	case err := <-errch:
-		return nil, err
+		return nil, NewTopLevelGenericErrWrapper(err)
 	}
 }
 
@@ -51,8 +51,8 @@ func CopyContext(ctx context.Context, dst io.Writer, src io.Reader) (int64, erro
 	case count := <-countch:
 		return count, nil
 	case <-ctx.Done():
-		return 0, ctx.Err()
+		return 0, NewTopLevelGenericErrWrapper(ctx.Err())
 	case err := <-errch:
-		return 0, err
+		return 0, NewTopLevelGenericErrWrapper(err)
 	}
 }