refactor(tracex): internally represent errors as strings

[bassosimone]

There are two reasons why this is beneficial:

1. github.com/google/go-cmp is more annoying to use for comparing
   data structures when there are interfaces to compare. Sure, there's
   a recipe for teaching it to compare errors, but how about making
   the errors trivially comparable instead?

2. if we want to send errors over the network, JSON serialization
   works but we cannot unmarshal the resulting string back to an error,
   so how about making this representation trivial to serialize (we
   are not going this now, but we need this property for websteps and
   it may be sensible to try to avoid to have duplicate code because
   of that -- measurex currently duplicates many tracex functionality
   and this is quite unfortunate because it slows development down)

Additionally, if an error is a string:

3. we can very easily use a switch for comparing its possible
   values with "" representing the absence of errors, while it is
   more complex to do the same when using a nullable string or even
   an error (i.e., an interface)

4. if a type is not nullable, it's easier to write safe code for
   it and we may want to refactor experiments to use the internal
   representation of measurements for more robust processing code

For all these reasons, let's internally use strings in tracex.

The overall aim here is to reduce the duplicated code between pre
and post-measurex measurements (see ooni/probe#2035).