Consider making the plugin stricter

[dmitry-timofeev]

Currently the plugin ignores any lines inside the codeinclude section that it does not recognize as include statements (also considering #10). It might result in various typos remaining undetected as the docs build successfully.

Possible issues with the permissive (current) approach:

• A totally invalid include statement is ignored, a snippet is skipped (likely, rare).
• An invalid targeting expression is ignored, a whole file is included instead of a particular part (more likely).
• No matches for a given syntactically correct targeting expression, a whole file is included instead (possibly, separate issue).