[chore] [cmd/schemagen] Fix pattern propagation and enhance error handling

[jkoronaAtCisco]

Description

PR provides fix for pattern propagation in schema resolver (when -r flag set) and improves overall error handling for better debugging.

Testing

Unit tests updated.

Authorship

• I, a human, wrote this pull request description myself.

[truthbk]

Hey @jkoronaAtCisco - if these are not urgent, would it be possible to wait for these contributions to go in, and maybe make the contribution at githu.com/open-telemetry/opentelemetry-collector? We're in the middle of moving schemagen to that repository with these two PRs:

• [cmd/schemagen] move schemagen tool from opentelemetry-collector-contrib opentelemetry-collector#15429
• [cmd/schemagen] remove schemagen tool, moved to opentelemetry-collector #48997

At the request of @dmitryax, to prevent circular references to the tool.

I hope that's OK. 🙏

Let me know if I can work with you, or help, to make it work.