feat!: More helpful error messages in model import (#2272)

## Duplicate of #2264, approved, wait to merge for breaking release.

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.

BREAKING CHANGE: The internals of ImportError are hidden.

---------

Co-authored-by: Lukas Heidemann <[email protected]>

Author: ss2165

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

@@ -80,3 +80,4 @@ proptest-derive = { workspace = true }
 # Required for documentation examples
 hugr = { path = "../hugr" }
 serde_yaml = "0.9.34"
+anyhow = { workspace = true }