refactor: merge dnsx and errorsx into netxlite (#517)

When preparing a tutorial for netxlite, I figured it is easier
to tell people "hey, this is the package you should use for all
low-level networking stuff" rather than introducing people to
a set of packages working together where some piece of functionality
is here and some other piece is there.

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

internal/netxlite/errorsx/errwrapper.go

@@ -1,100 +0,0 @@
-package errorsx
-
-import "encoding/json"
-
-// ErrWrapper is our error wrapper for Go errors. The key objective of
-// this structure is to properly set Failure, which is also returned by
-// the Error() method, to be one of the OONI failure strings.
-//
-// OONI failure strings are defined in the github.com/ooni/spec repo
-// at https://github.com/ooni/spec/blob/master/data-formats/df-007-errors.md.
-type ErrWrapper struct {
-	// Failure is the OONI failure string. The failure strings are
-	// loosely backward compatible with Measurement Kit.
-	//
-	// This is either one of the FailureXXX strings or any other
-	// string like `unknown_failure: ...`. The latter represents an
-	// error that we have not yet mapped to a failure.
-	Failure string
-
-	// Operation is the operation that failed.
-	//
-	// New code will always nest ErrWrapper and you need to
-	// walk the chain to find what happened.
-	//
-	// The following comment describes the DEPRECATED
-	// legacy behavior implements by internal/engine/legacy/errorsx:
-	//
-	// If possible, the Operation string
-	// SHOULD be a _major_ operation. Major operations are:
-	//
-	// - ResolveOperation: resolving a domain name failed
-	// - ConnectOperation: connecting to an IP failed
-	// - TLSHandshakeOperation: TLS handshaking failed
-	// - HTTPRoundTripOperation: other errors during round trip
-	//
-	// Because a network connection doesn't necessarily know
-	// what is the current major operation we also have the
-	// following _minor_ operations:
-	//
-	// - CloseOperation: CLOSE failed
-	// - ReadOperation: READ failed
-	// - WriteOperation: WRITE failed
-	//
-	// If an ErrWrapper referring to a major operation is wrapping
-	// another ErrWrapper and such ErrWrapper already refers to
-	// a major operation, then the new ErrWrapper should use the
-	// child ErrWrapper major operation. Otherwise, it should use
-	// its own major operation. This way, the topmost wrapper is
-	// supposed to refer to the major operation that failed.
-	Operation string
-
-	// WrappedErr is the error that we're wrapping.
-	WrappedErr error
-}
-
-// Error returns a description of the error that occurred.
-func (e *ErrWrapper) Error() string {
-	return e.Failure
-}
-
-// Unwrap allows to access the underlying error
-func (e *ErrWrapper) Unwrap() error {
-	return e.WrappedErr
-}
-
-// MarshalJSON converts an ErrWrapper to a JSON value.
-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,
-	}
-}
-
-// NewTopLevelGenericErrWrapper wraps an error occurring at top
-// level using the most generic available classifier.
-func NewTopLevelGenericErrWrapper(err error) *ErrWrapper {
-	return NewErrWrapper(ClassifyGenericError, TopLevelOperation, err)
-}