feat: More helpful error messages in model import (#2264)

This PR adds more helpful error messages to model import to help debug
errors that may occur during the migration to `hugr-model` as the
default serialisation format (see #2260). The errors are still far from
ideal, but should already provide a significant improvement. We hide the
internals of `ImportError` in order to allow for the error messages to
improve without incurring breaking changes. This PR also implements a
remaining `todo!()` concerning importing tuple types.

Author: zrho

Cargo.lock

@@ -42,6 +42,7 @@ result_large_err = "allow"
 large_enum_variant = "allow"
 
 [workspace.dependencies]
+anyhow = "1.0.98"
 insta = { version = "1.43.1" }
 bitvec = "1.0.1"
 capnp = "0.20.6"

Cargo.toml

@@ -77,3 +77,4 @@ proptest-derive = { workspace = true }
 # Required for documentation examples
 hugr = { path = "../hugr" }
 serde_yaml = "0.9.34"
+anyhow = { workspace = true }