notify before using LSP as fallback formatter

I want to know about the configured formatter failing, because the LSP's
formatting behavior might be different, and I might not notice the
difference.

I've also added a comment to illustrate what other options are possible,
such as turning off the fallback entirely with `false`. I'll leave the
fallback up for now because at least I'm getting notifications!

This was motivated by initially being confused about whether `mix` was
trimming trailing whitespace or not. Turns out it *was* trimming
trailing whitespace, just not for comments.

Author: TheDDMbassy

lua/plugins/conform.lua

@@ -13,7 +13,7 @@ return { -- Autoformat
     },
   },
   opts = {
-    notify_on_error = false,
+    notify_on_error = true,
     format_on_save = function(bufnr)
       -- Disable "format_on_save lsp_fallback" for languages that don't
       -- have a well standardized coding style. You can add additional
@@ -24,7 +24,7 @@ return { -- Autoformat
       else
         return {
           timeout_ms = 500,
-          lsp_format = 'fallback',
+          lsp_format = 'fallback', -- or false
         }
       end
     end,