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/mocks/logger.go

@@ -0,0 +1,18 @@
+package mocks
+
+// Logger allows mocking a logger.
+type Logger struct {
+	MockDebug func(message string)
+
+	MockDebugf func(format string, v ...interface{})
+}
+
+// Debug calls MockDebug.
+func (lo *Logger) Debug(message string) {
+	lo.MockDebug(message)
+}
+
+// Debugf calls MockDebugf.
+func (lo *Logger) Debugf(format string, v ...interface{}) {
+	lo.MockDebugf(format, v...)
+}