Partially fix spec validation

[FabijanC]

Summary

• Close Validation script not applied correctly #252
  • Ignoring starknet_metadata.json, as was already the case.
  • Using xargs instead of the -exec feature of find to allow propagating non-zero exit code and exiting in case of validation failure.
• Close Validation script not exiting correctly in case of finding an error #253
• Close Duplicate error definitions #254
  • Introduce a new script in package.json that is used as a new CI step.

Notes

• Supersedes Fix CI and spec versions #182.
• Someone please mark the issues as closeable by this PR (seems like I don't have the rights to do so).
• It would probably make sense to target this PR into master, but since Add format script and run it #241 is targeting v0.8.0, I did the same.
• The title says the fix is partial because I do not guarantee the underlying validation is complete.