test/functional: retry/Screen: failure instead of error #11173

- Running out of retries, or unexpected screen state should make the
  test FAIL, not ERROR.
- Uses levels to report the location of the caller.
- Improve message with retry-failure (formatting).

Before:

    [ RUN      ] test: 103.53 ms ERR
    test/functional/helpers.lua:388:
    retry() attempts: 1
    test/functional/ui/screen.lua:587: Row 1 did not match.
    Expected:
      |*X^                         |
      |{0:~                        }|
      |{0:~                        }|
      |                         |
    Actual:
      |*^                         |
      |{0:~                        }|
      |{0:~                        }|
      |                         |

    To print the expect() call that would assert the current screen state, use
    screen:snapshot_util(). In case of non-deterministic failures, use
    screen:redraw_debug() to show all intermediate screen states.

    stack traceback:
            test/functional/helpers.lua:388: in function 'retry'
            test/functional/test_spec.lua:24: in function <test/functional/test_spec.lua:23>

After:

    [ RUN      ] test: 105.22 ms FAIL
    test/functional/test_spec.lua:24: stopping after 1 retry() attempts.
    test/functional/test_spec.lua:25: Row 1 did not match.
    Expected:
      |*X^                         |
      |{0:~                        }|
      |{0:~                        }|
      |                         |
    Actual:
      |*^                         |
      |{0:~                        }|
      |{0:~                        }|
      |                         |

    To print the expect() call that would assert the current screen state, use
    screen:snapshot_util(). In case of non-deterministic failures, use
    screen:redraw_debug() to show all intermediate screen states.

    stack traceback:
            test/functional/helpers.lua:389: in function 'retry'
            test/functional/test_spec.lua:24: in function <test/functional/test_spec.lua:23>

test/functional/helpers.lua

@@ -1,4 +1,5 @@
 require('coxpcall')
+local busted = require('busted')
 local luv = require('luv')
 local lfs = require('lfs')
 local mpack = require('mpack')
@@ -385,7 +386,7 @@ function module.retry(max, max_ms, fn)
     end
     luv.update_time()  -- Update cached value of luv.now() (libuv: uv_now()).
     if (max and tries >= max) or (luv.now() - start_time > timeout) then
-      error("\nretry() attempts: "..tostring(tries).."\n"..tostring(result))
+      busted.fail(string.format("retry() attempts: %d\n%s", tries, tostring(result)), 2)
     end
     tries = tries + 1
     luv.sleep(20)  -- Avoid hot loop...

test/functional/ui/screen.lua

@@ -71,6 +71,7 @@
 -- To debug screen tests, see Screen:redraw_debug().
 
 local helpers = require('test.functional.helpers')(nil)
+local busted = require('busted')
 local deepcopy = helpers.deepcopy
 local shallowcopy = helpers.shallowcopy
 local concat_tables = helpers.concat_tables
@@ -574,7 +575,7 @@ asynchronous (feed(), nvim_input()) and synchronous API calls.
 
 
   if err then
-    assert(false, err)
+    busted.fail(err, 3)
   elseif did_warn then
     local tb = debug.traceback()
     local index = string.find(tb, '\n%s*%[C]')