Improve diagnostics for files with named code paths

[alexander-yevsyukov]

Currently, the error message looks like this:

Panic: failed to embed code fragment into doc file `file:///Users/sanders/Projects/Spine/validation/docs/content/docs/validation/04-validators/implement-a-validator.md:53`: code file `$runtime/src/main/kotlin/io/spine/validation/TimestampValidator.kt` not found

To understand the nature of the problem it takes to expand $runtime/src/main/kotlin/io/spine/validation/TimestampValidator.kt using the value of $runtime in the programmer's head. We need to make it more convenient for the users.

If the prefixed path is not found, please give the prefixed value and the value into which it computes.