Improve the display of missing files

[Vladyslav-Kuksiuk]

This PR improves displaying of paths, when file or fragment wasn't found.
Resolves this issue.

Possible errors:

• When named path wasn't found:

Panic: failed to embed code fragment into doc file `file:///Users/vladyslav.kuksiuk/Desktop/Work/embed-code-go/my-test/target/target.md:2`: code root with name `first1` not found for path `$first1/hello/Hello.kt`

• When source file was found, but fragment wasn't:

Panic: failed to embed code fragment into doc file `file:///Users/vladyslav.kuksiuk/Desktop/Work/embed-code-go/my-test/target/target.md:2`: fragment `main1` from code file `file:///Users/vladyslav.kuksiuk/Desktop/Work/embed-code-go/my-test/source/hello/Hello.kt` not found

• When source file was not found, by named path:

Panic: failed to embed code fragment into doc file `file:///Users/vladyslav.kuksiuk/Desktop/Work/embed-code-go/my-test/target/target.md:2`: fragment `main` from code file `$first/hello/Hello2.kt (/Users/vladyslav.kuksiuk/Desktop/Work/embed-code-go/my-test/source/hello/Hello2.kt)` not found

• When file was not found, by unnamed path:

Panic: failed to embed code fragment into doc file `file:///Users/vladyslav.kuksiuk/Desktop/Work/embed-code-go/my-test/target/target.md:2`: fragment `main` from code file `/Users/vladyslav.kuksiuk/Desktop/Work/embed-code-go/my-test/source/hello/Hello2.kt` not found

[dmytro-kashcheiev]

@Vladyslav-Kuksiuk LGTM with few comments to check before merge.

[dmytro-kashcheiev]

The method by itself is a helper for logging, maybe it's not a best idea to panic if we can't formate logs correctly.

[dmytro-kashcheiev]

Same as above.