Tell users when a fixer does not exist, and make the no fixers message softer

autoload/ale/fix.vim

@@ -352,11 +352,21 @@ function! ale#fix#Fix(...) abort
         throw "fixing_flag must be either '' or 'save_file'"
     endif
 
-    let l:callback_list = s:GetCallbacks()
+    try
+        let l:callback_list = s:GetCallbacks()
+    catch /E700/
+        let l:function_name = join(split(split(v:exception, ':')[3]))
+        echom printf(
+        \   'There is no fixer named `%s`. Check :ALEFixSuggest',
+        \   l:function_name,
+        \)
+
+        return 0
+    endtry
 
     if empty(l:callback_list)
         if l:fixing_flag is# ''
-            echoerr 'No fixers have been defined. Try :ALEFixSuggest'
+            echom 'No fixers have been defined. Try :ALEFixSuggest'
         endif
 
         return 0