plugin: handle sandbox mode gracefully

When the sandbox is active (e.g. while debugging something), certains
commands and expressions are not allowed (see :h sandbox) and may cause
nasty exceptions to be thrown.

So try to catch E48 and handle it gracefully

autoload/airline/util.vim

@@ -180,7 +180,12 @@ function! airline#util#doautocmd(event)
     " airline disabled
     return
   endif
-  exe printf("silent doautocmd %s User %s", s:nomodeline, a:event)
+  try
+    exe printf("silent doautocmd %s User %s", s:nomodeline, a:event)
+  catch /^Vim\%((\a\+)\)\=:E48:/
+    " Catch: Sandbox mode
+    " no-op
+  endtry
 endfunction
 
 function! airline#util#themes(match)