Smithy4sThrowable toString ignores fields if message is present

[kubukoz]

When you generate code from this:

$version: "2"

namespace input

@error("client")
structure Foo {
  @smithy4s.meta#errorMessage message: String
  @required extra: String
} 

if message is present, you won't see extra:

println(input.Foo(extra = "important details", message = Some("failure!")))
// input.Foo: failure!

println(input.Foo(extra = "important details", message = None))
// input.Foo(important details, None)

This can be really confusing, e.g. in tests that are based on equals + toString, which is the default Weaver behavior if you don't have a Comparison instance - your test will fail, but you won't see a diff, because only the message is part of the toString.

[kubukoz]

I think we should expand the handling of this else branch:

smithy4s/modules/core/src/smithy4s/Smithy4sThrowable.scala

Lines 34 to 36 in e328afb

	} else {
	s"$name: $message"
	}

so that it includes the other fields as well.

[stewSquared]

I think I can take this one on.

[stewSquared]

So, suppose we extend s"$name: $message"

Would we still want to give the message field special treatment in formatting? It seems reasonable to be to render as:

input.Foo(important details, Some(failure!))

without the : $message at the end. Alternatively, we could make it more descriptive with filed names, like so:

input.Foo(extra = "important details",  message = Some("failure!"))

I'm inclined to go with the latter, since it labels the intended message. In either case, there would also no longer be a need for separate branches. I'll probably go ahead with this, but appreciate thoughts if you have them.

edit: Well, message isn't necessarily derived from a field of a case class extending this, so I'm doing what I said above, but keeping : $message at the end.

[kubukoz]

My vote would be to not add field names just yet.

This sound good?

// in Some case
input.Foo(important details, Some(failure!)): failure!

// in None case - equivalent to not using `@errorMessage` at all
input.Foo(important details, None)