[TASK] Improve error message in blueprint-metadata

[drewstone]

Overview

When running blueprint_metadata::generate_json(), the rustdoc JSON output is expected to match that of the version in the rust-toolchain.toml in gadget.

When building a blueprint, however, since it also has a rust-toolchain.toml, the doc output will match whatever that version happens to be, leading to this confusion error:

  Reading JSON from /Users/drew/webb/discord-summarizer-rig-blueprint/target/blueprint/doc/discord_summarizer_rig_blueprint.json

  thread 'main' panicked at /Users/drew/.cargo/git/checkouts/gadget-84e3289516f70bdc/b40601a/crates/blueprint/metadata/src/lib.rs:462:59:
  Failed to parse rustdoc JSON: Error("missing field `path`", line: 1, column: 410)
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

We should give a nice error message when deserialization fails, pointing users to the rust-toolchain.toml file in gadget, and telling them that they must match that version.