Update ProcessMonitor errors to contain error messages

[chalcolith]

This is a breaking change, as the ProcessError types are now classes, not primitives, and have a .string() method to return their error messages.

In addition, there is no more Unsupported error type.

[SeanTAllen]

@kulibali I dont think these should be classes.

They should stay as primitives but primitives with a "string" function that returns an error message. The string representation of the error.

[chalcolith]

I thought it would be useful to be able to be able to include, e.g. particular filenames (or Windows error messages) in the strings.

Do you think this is not worth it?

[SeanTAllen]

@kulibali I'm not sure of the utility right now based on what I know about the usage.

[chalcolith]

I would find it quite useful for debugging my Windows updates to corral, stable, and ponyup.

[SeanTAllen]

@kulibali perhaps a mix of class and primitive? Do they all need a custom message, for corral, its a string that was just "blah" basically for a string representation of the name. I suppose classes don't really matter much as we won't be allocating a lot of them.

[SeanTAllen]

@kulibali thinking about it some more. if it is going to be classes. I think a generic error class that contains both a possible custom error and the primitive for type of error would be best with each primitive having its own default string representation.

Then it could be something like (this shortened version)

class ProcessError
  let error_type: ProcessErrorType
  let message: String

fun string(): String =>
  prim.string() + ": " + message

(but more allocation friendly) for string().

Thoughts?

[mfelsche]

😻

[mfelsche]

Having the string() function return a String val might be confusing, as Stringable in stdlib is defined as returning a String iso^. I would also make the ProcessErrorType primitives return a String iso^ to be in line with Stringable as well.

May I suggest to use sth like this in this line here?

let err_type_name = error_type.string()
let msg = 
  recover iso 
    let tmp = String(err_type_name.size() + 2 + m.size())
    tmp.>append(consume err_type_name)
    .>append(": ")
    .>append(m)
end

[chalcolith]

I was thinking that the returned string should not be mutable.

[SeanTAllen]

I think String iso^ is preferable so it lines up with Stringable.

[SeanTAllen]

Being Stringable was my intent when I suggested the idea.

[chalcolith]

OK, updated.