fix: show contextual errors for failed GitHub package.json scans

[t128n]

Linked issue

Resolves #70

Context

GitHub scans previously returned an empty string when no package.json could be resolved. That value was then parsed as JSON, causing the UI to report File was not valid JSON. even though no file had been found.

Description

This PR adds two error modes that previously produced the same validation error:

Scenario	Message
No package.json can be found in the GitHub repository or selected path	package.json was not found in this GitHub repository or selected path.
A package.json is found but contains malformed JSON	package.json was found in this GitHub repository or selected path, but was not valid JSON.

Before/After

Scenario	URL	Prod	PR
package.json not found	replacements.fyi/package-json?owner=t128n&repo=not-found
Invalid package.json	replacements.fyi/package-json?owner=t128n&repo=invalid-package-json