[RDY] Remove more OOM error handling code

 * Removed code from more functions that cal any of `*alloc()`
  - ExpandBufnames
  - buf_modname()
  - do_autocmd_event()
  - ff_create_stack_element()
  - ff_get_visited_list()
  - ins_complete()
  - msg_show_console_dialog()
  - prt_find_resource()
  - vim_findfile_init()
 * After #466 many functions don't need to return error codes. I went through calls to some of these functions (e.g. `ga_grow()`) and removed the now unnecessary error handling code.
  - diff_alloc_new()
  - transstr()
  - mf_hash_grow()
  - ga_grow()
  - foldInsert()
  - alist_add_list()
  - get_arg_list()
  - store_loop_line()
  - push_current_state()

call for reviewers: @schmee @lslah

This PR is much easier to review commit by commit.