patch 8.0.1599: no error message when gdb does not support debugger

Problem:    No error message when gdb does not support the terminal debugger.
Solution:   Check for the response to open the Machine Interface.

runtime/pack/dist/opt/termdebug/plugin/termdebug.vim

@@ -105,9 +105,39 @@ func s:StartDebug(cmd)
  let s:gdbwin = win_getid(winnr())

  " Connect gdb to the communication pty, using the GDB/MI interface
  " If you get an error "undefined command" your GDB is too old.
  call term_sendkeys(s:gdbbuf, 'new-ui mi ' . commpty . "\r")

  " Wait for the response to show up, users may not notice the error and wonder
  " why the debugger doesn't work.
  let try_count = 0
  while 1
    let response = ''
    for lnum in range(1,20)
      if term_getline(s:gdbbuf, lnum) =~ 'new-ui mi '
	let response = term_getline(s:gdbbuf, lnum + 1)
	if response =~ 'Undefined command'
	  echoerr 'Your gdb does not support the Machine Interface feature'
	  exe 'bwipe! ' . s:ptybuf
	  exe 'bwipe! ' . s:commbuf
	  return
	endif
	if response =~ 'New UI allocated'
	  " Success!
	  break
	endif
      endif
    endfor
    if response =~ 'New UI allocated'
      break
    endif
    let try_count += 1
    if try_count > 100
      echoerr 'Cannot check if your gdb works, continuing anyway'
      break
    endif
    sleep 10m
  endwhile

  " Interpret commands while the target is running.  This should usualy only be
  " exec-interrupt, since many commands don't work properly while the target is
  " running.

src/version.c

@@ -766,6 +766,8 @@ static char *(features[]) =

static int included_patches[] =
{   /* Add new patch number below this line */
/**/
    1599,
/**/
    1598,
/**/