Handle linter callback functions being unknown or deleted

autoload/ale/engine.vim

@@ -222,7 +222,12 @@ function! s:HandleExit(job_id, exit_code) abort
         call ale#history#RememberOutput(l:buffer, a:job_id, l:output[:])
     endif
 
+    try
         let l:loclist = ale#util#GetFunction(l:linter.callback)(l:buffer, l:output)
+    " Handle the function being unknown, or being deleted.
+    catch /E700/
+        let l:loclist = []
+    endtry
 
     call ale#engine#HandleLoclist(l:linter.name, l:buffer, l:loclist)
 endfunction

doc/ale.txt

@@ -2205,6 +2205,10 @@ ale#linter#Define(filetype, linter)                       *ale#linter#Define()*
                          defined, as LSP linters handle diagnostics
                          automatically. See |ale-lsp-linters|.
 
+                         If the function named does not exist, including if
+                         the function is later deleted, ALE will behave as if
+                         the callback returned an empty list.
+
                          The keys for each item in the List will be handled in
                          the following manner:
                                                            *ale-loclist-format*