Fail textDocument/formatting when no formatter is set (#108)

Currently, nil silently fails when no formatter is set. This caused me a
lot of confusion as to why nothing was formatting when I accidentally
misconfigured the formatter.

This makes it so that we return an error instead, alerting the user that
something is wrong.

This could be annoying to someone who hasn't configured a formatter on
purpose but still causes formatting events, either by format-on-save or
muscle memory. I think this is fine, and they should turn off
format-on-save or just get a formatter instead. Alternatively, someone
could set `cat` as their formatter.

crates/nil/src/handler.rs

@@ -221,9 +221,10 @@ pub(crate) fn formatting(
         Ok(stdout)
     }
 
-    let Some(cmd) = &snap.config.formatting_command else {
-        return Ok(None);
-    };
+    let cmd =
+        snap.config.formatting_command.as_ref().context(
+            "No formatter configured. Set the nil.formatting.command LSP server setting.",
+        )?;
 
     let (file_content, line_map) = {
         let vfs = snap.vfs();