Improve error output of the embedding instruction parser

[Vladyslav-Kuksiuk]

This PR improves the error output of the embedding instruction parser:

• Error when start/end pattern cannot be found:

  <embed-code file="$test/Hello.kt" start="not existing pattern">
  </embed-code>
  ```kotlin
  ```
  

  
• Error when markdown fence is not started/finished after embedding instruction
  
  

Resolves this issue

[dmytro-kashcheiev]

@Vladyslav-Kuksiuk LGTM