Use Core.Error.tag/tag_arg when adding info to errors

[mrmr1993]

Currently, we do a variety of different things to create an error with additional information out of an Error.t. Most of these involve converting the Error.t to a string with Error.to_string_hum and then creating a new exception with failwithf, Error.createf, Error.of_string, etc. This way, we end up with errors which are s-expressions encoded as strings in s-expressions encoded as strings etc.

For example, in coda_net2 we have

      failwithf "unexpected error doing setGatingConfig: %s"
        (Error.to_string_hum e) ()

Core has functions Error.tag : Error.t -> tag:string -> Error.t and Error.tag_arg : Error.t -> string -> 'a -> ('a -> Sexp.t) -> Error.t designed for adding this information without disrupting or string-encoding the underlying data. These are designed for this purpose, and we should use them!

The above example could easily be changed to

Error.raise (Error.tag ~tag:"unexpected error doing setGatingConfig" e)

This has the added advantage of working nicely with #6698, which will accumulate these tags and print them as part of the JSON metadata that we log.