fix(exceptions): restore did_throw (#20000)

`!did_throw` doesn't exactly imply `!current_exception`, as `did_throw = false`
is sometimes used to defer exception handling for later (without forgetting the
exception). E.g: uncaught exception handling in `do_cmdline()` may be deferred
to a different call (e.g: when `try_level > 0`).

In #7881, `current_exception = NULL` in `do_cmdline()` is used as an analogue of
`did_throw = false`, but also causes the pending exception to be lost, which
also leaks as `discard_exception()` wasn't used.

It may be possible to fix this by saving/restoring `current_exception`, but
handling all of `did_throw`'s edge cases seems messier. Maybe not worth
diverging over.

This fix also uncovers a `man_spec.lua` bug on Windows: exceptions are thrown
due to Windows missing `man`, but they're lost; skip these tests if `man` isn't
executable.

test/functional/plugin/man_spec.lua

@@ -6,6 +6,12 @@ local funcs = helpers.funcs
 local nvim_prog = helpers.nvim_prog
 local matches = helpers.matches
 
+clear()
+if funcs.executable('man') == 0 then
+  pending('missing "man" command', function() end)
+  return
+end
+
 describe(':Man', function()
   before_each(function()
     clear()