Improve error diagnostics when checking embedding

[alexander-yevsyukov]

Currently, the error message looks like this:

Failed to render ./docs/guides/integration.md:103: Fragment file `/home/runner/work/SpineEventEngine.github.io/SpineEventEngine.github.io/.fragments/examples/airport/airplane-supplies/src/main/proto/spine/example/airport/supplies/supplies_service.proto` not found. This may indicate that the file is not includedin `code_includes` or is not readable.

The file is included into the embed_code section of _config.yml using the **/*.proto syntax.
The file is also there and is readable when checked macOS when developing the site.

The subject of this issue is not fixing the issue, but improving the diagnostic when something wrong happens with accessing the file. Currently it says that the problem is either “this” or “that”. We need to tell exactly what's the problem so that the user can fix it quicker.

A minor issue is that the space is missing in “includedin” before “in”.

It is still puzzling for me why we need code_includes configuration when we reference the files explicitly when embedding them into markdown.