patch 9.1.0529: silent! causes following try/catch to not work

Problem:  silent! causes following try/catch to not work
          (Malcolm Rowe)
Solution: consider emsg_silent in handle_did_throw() and do not abort
          evaluation flow for :silent! (LemonBoy)

The silent! flag causes the evaluation not to be aborted in case of
uncaught exceptions, adjust handle_did_throw to take this detail into
account.

Fixes the long-standing todo.txt item:
```
Problem that a previous silent ":throw" causes a following try/catch not
to work. (ZyX, 2013 Sep 28) With examples: (Malcolm Rowe, 2015 Dec 24)
Also see #8487 for an example.
```

fixes: #538
closes: #15128

Signed-off-by: LemonBoy <thatlemon@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/todo.txt

@@ -1,4 +1,4 @@
-*todo.txt*      For Vim version 9.1.  Last change: 2024 Jun 03
+*todo.txt*      For Vim version 9.1.  Last change: 2024 Jul 04
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -512,10 +512,6 @@ Information for a specific terminal (e.g. gnome, tmux, konsole, alacritty) is
 spread out.  Make a section with copy/paste examples of script and pointers to
 more information.
 
-Problem that a previous silent ":throw" causes a following try/catch not to
-work. (ZyX, 2013 Sep 28) With examples: (Malcolm Rowe, 2015 Dec 24)
-Also see #8487 for an example.
-
 Request to use "." for the cursor column in search pattern \%<.c and \%<.v.
 (#8179)
 

src/ex_docmd.c

@@ -1460,8 +1460,13 @@ handle_did_throw(void)
     current_exception->throw_name = NULL;
 
     discard_current_exception();	// uses IObuff if 'verbose'
-    suppress_errthrow = TRUE;
+
-    force_abort = TRUE;
+    // If "silent!" is active the uncaught exception is not fatal.
+    if (emsg_silent == 0)
+    {
+	suppress_errthrow = TRUE;
+	force_abort = TRUE;
+    }
 
     if (messages != NULL)
     {

src/testdir/test_vimscript.vim

@@ -7536,6 +7536,31 @@ func Test_deeply_nested_source()
   call system(cmd)
 endfunc
 
+func Test_exception_silent()
+  XpathINIT
+  let lines =<< trim END
+  func Throw()
+    Xpath 'a'
+    throw "Uncaught"
+    " This line is not executed.
+    Xpath 'b'
+  endfunc
+  " The exception is suppressed due to the presence of silent!.
+  silent! call Throw()
+  try
+    call DoesNotExist()
+  catch /E117:/
+    Xpath 'c'
+  endtry
+  Xpath 'd'
+  END
+  let verify =<< trim END
+    call assert_equal('acd', g:Xpath)
+  END
+
+  call RunInNewVim(lines, verify)
+endfunc
+
 "-------------------------------------------------------------------------------
 " Modelines								    {{{1
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    529,
 /**/
     528,
 /**/