Echo the command to be executed when using `:verbose Ag`
handle pattern starting with quote then 1 or more spaces

fixes #9
fix ag error handling

- on no results ag returns an error code, so only attempt to echo an
  error if the error message is not empty, otherwise assume no results
- for filename searches (-g) ag (0.24.1) is returning an error code even
  if results were found, so account for that.
Check against `2` with `exists` for commands

Ref: https://github.com/tomtom/tplugin_vim/issues/9
use word-under-cursor for call without argument