Add thiserror instead of exposing eyre anonymous errors

The change here is to make it easier for the consumer to debug the api.
Such that they can `match` on individual errors instead of having to
parse text.

eyre is convenient, but mostly from a consumers perspective

crates/dagger-core/Cargo.toml

@@ -16,6 +16,7 @@ serde_json = { workspace = true }
 tokio = { workspace = true }
 tracing = { workspace = true }
 tracing-subscriber = { workspace = true }
+thiserror.workspace = true
 
 base64 = "0.21.0"
 dirs = "4.0.0"