Improve error message when CodeReference snippet is not found (#982)

When using (*** include:name ***) with a misspelled or undefined snippet
name, the previous code threw a cryptic KeyNotFoundException. Now it
throws a clear message indicating which snippet name was not found and
how to define it.

Fixes #673

Co-authored-by: Repo Assist <repo-assist@github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

src/FSharp.Formatting.Literate/Transformations.fs

@@ -514,7 +514,14 @@ module internal Transformations =
                         match special with
                         | RawBlock(lines, _) -> Some(InlineHtmlBlock(unparse lines, None, None))
                         | LiterateCode(lines, _, _) -> Some(formatted.[Choice1Of2 lines])
-                        | CodeReference(ref, _) -> Some(formatted.[Choice2Of2 ref])
+                        | CodeReference(ref, _) ->
+                            match formatted.TryGetValue(Choice2Of2 ref) with
+                            | true, v -> Some v
+                            | false, _ ->
+                                failwithf
+                                    "Could not find named code snippet '%s'. Check that it is defined with '(** define:%s ***)'."
+                                    ref
+                                    ref
                         | FsiMergedOutputReference _
                         | FsiOutputReference _
                         | OutputReference _