refactor(netxlite/errors): improve docs and format code (#481)

No real functional change. A few are needed and they will come
next. With this diff I just wanted to do cosmetic changes and
documentation changes, to ensure this package is okay.

See https://github.com/ooni/probe/issues/1591

internal/netxlite/errorsx/doc.go

@@ -1,2 +1,20 @@
 // Package errorsx contains code to classify errors.
+//
+// We define the ErrWrapper type, that should wrap any error
+// and map it to the corresponding OONI failure.
+//
+// See https://github.com/ooni/spec/blob/master/data-formats/df-007-errors.md
+// for a list of OONI failure strings.
+//
+// We define ClassifyXXX functions that map an `error` type to
+// the corresponding OONI failure.
+//
+// When we cannot map an error to an OONI failure we return
+// an "unknown_failure: XXX" string where the XXX part has
+// been scrubbed so to remove any network endpoints.
+//
+// The general approach we have been following for this
+// package has been to return the same strings that we used
+// with the previous measurement engine, Measurement Kit
+// available at https://github.com/measurement-kit/measurement-kit.
 package errorsx