Remember fmt failure on a per-buffer basis

Fixes #720

autoload/go/fmt.vim

@@ -43,8 +43,6 @@ if !exists("g:go_fmt_experimental")
     let g:go_fmt_experimental = 0
 endif
 
-let s:got_fmt_error = 0
-
 "  we have those problems : 
 "  http://stackoverflow.com/questions/12741977/prevent-vim-from-updating-its-undo-tree
 "  http://stackoverflow.com/questions/18532692/golang-formatter-and-vim-how-to-destroy-history-record?rq=1
@@ -120,13 +118,13 @@ function! go#fmt#Format(withGoimport)
         let &fileformat = old_fileformat
         let &syntax = &syntax
 
-        " clean up previous location list, but only if it's due fmt
-        if s:got_fmt_error 
-            let s:got_fmt_error = 0
+        " clean up previous location list, but only if it's due to fmt
+        if exists('b:got_fmt_error') && b:got_fmt_error
+            let b:got_fmt_error = 0
             call go#list#Clean(l:listtype)
             call go#list#Window(l:listtype)
         endif
-    elseif g:go_fmt_fail_silently == 0 
+    elseif g:go_fmt_fail_silently == 0
         let splitted = split(out, '\n')
         "otherwise get the errors and put them to location list
         let errors = []
@@ -147,7 +145,7 @@ function! go#fmt#Format(withGoimport)
             echohl Error | echomsg "Gofmt returned error" | echohl None
         endif
 
-        let s:got_fmt_error = 1
+        let b:got_fmt_error = 1
         call go#list#Window(l:listtype, len(errors))
 
         " We didn't use the temp file, so clean up