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.

src/nvim/eval/userfunc.c

@@ -3010,7 +3010,7 @@ void ex_call(exarg_T *eap)
 
   // When inside :try we need to check for following "| catch" or "| endtry".
   // Not when there was an error, but do check if an exception was thrown.
-  if ((!aborting() || current_exception != NULL) && (!failed || eap->cstack->cs_trylevel > 0)) {
+  if ((!aborting() || did_throw) && (!failed || eap->cstack->cs_trylevel > 0)) {
     // Check for trailing illegal characters and a following command.
     if (!ends_excmd(*arg)) {
       if (!failed && !aborting()) {