feat(blueprint-metadata): nicer errors

[Serial-ATA]

This only adds error messages for some cases. Many others are still just panics.

This also fixes the issue of missing metadata producing an empty BlueprintMetadata and allowing the crate to continue building. It's now a hard error.

Removing the metadata from incredible-squaring yields:

warning: incredible-squaring-blueprint@0.1.1: Failed to generate blueprint metadata: Blueprint metadata not found in the Cargo.toml
error: failed to run custom build command for `incredible-squaring-blueprint v0.1.1 (<path>/blueprints/incredible-squaring)`

Caused by:
  process didn't exit successfully: `<path>/target/debug/build/incredible-squaring-blueprint-a1eaf06037a611fc/build-script-build` (exit status: 1)
  --- stdout
  cargo:rerun-if-changed=src/lib.rs
  cargo:rerun-if-changed=src/main.rs
  Adding test fetcher since none exists
  cargo:warning=Failed to generate blueprint metadata: Blueprint metadata not found in the Cargo.toml

  --- stderr
  Reading JSON from <path>/blueprints/incredible-squaring/target/blueprint/doc/incredible_squaring_blueprint.json
  [INFO] Extracted 1 job definitions
  [WARN] No gadget metadata found in the Cargo.toml.
  [WARN] For more information, see: <TODO>
  [ERROR]: No blueprint metadata found in the Cargo.toml.
  [ERROR]: For more information, see: <TODO>

Notice the new cargo warning message up top and the log levels in stderr. Still isn't that pretty, but it's the best that can be done AFAIK.