patch 8.1.2103: wrong error message if "termdebugger" is not executable

Problem:    wrong error message if "termdebugger" is not executable.
Solution:   Check if "termdebugger" is executable and give a clear error
            message. (Ozaki Kiichi, closes #5000)  Fix indents.

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

@@ -65,8 +65,8 @@ command -nargs=* -complete=file -bang Termdebug call s:StartDebug(<bang>0, <f-ar
 command -nargs=+ -complete=file -bang TermdebugCommand call s:StartDebugCommand(<bang>0, <f-args>)
 
 " Name of the gdb command, defaults to "gdb".
-if !exists('termdebugger')
-  let termdebugger = 'gdb'
+if !exists('g:termdebugger')
+  let g:termdebugger = 'gdb'
 endif
 
 let s:pc_id = 12
@@ -104,9 +104,14 @@ endfunc
 
 func s:StartDebug_internal(dict)
   if exists('s:gdbwin')
-    echoerr 'Terminal debugger already running'
+    echoerr 'Terminal debugger already running, cannot run two'
     return
   endif
+  if !executable(g:termdebugger)
+    echoerr 'Cannot execute debugger program "' .. g:termdebugger .. '"'
+    return
+  endif
+
   let s:ptywin = 0
   let s:pid = 0
 

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2103,
 /**/
     2102,
 /**/