util: provide some useful output strings

autoload/go/util.vim

@@ -193,23 +193,21 @@ function! go#util#Exec(cmd, ...) abort
  " CheckBinPath will show a warning for us.
  let l:bin = go#path#CheckBinPath(l:bin)
  if empty(l:bin)
- return ['', 1]
+ return ['command not found', 1]
  endif
 
  " Finally execute the command using the full, resolved path. Do not pass the
  " unmodified command as the correct program might not exist in $PATH.
  try
  return call('s:exec', [[l:bin] + a:cmd[1:]] + a:000)
  catch
- " TODO(bc): return v:exception as the output here or write it with
- " go#util#EchoError?
- return ['', 1]
+ return [v:exception, 1]
  endtry
 endfunction
 
 function! go#util#ExecInDir(cmd, ...) abort
  if !isdirectory(expand("%:p:h"))
- return ['', 1]
+ return ['not a directory', 1]
  endif
 
  let cd = exists('*haslocaldir') && haslocaldir() ? 'lcd ' : 'cd '