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>

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