improved server subprocess launch failure error handling

assets/src/server/ballisticacore_server.py

@@ -583,13 +583,20 @@ class ServerManagerApp:
         binary_name = ('ballisticacore_headless.exe'
                        if os.name == 'nt' else './ballisticacore_headless')
         assert self._ba_root_path is not None
+        self._subprocess = None
+        try:
             self._subprocess = subprocess.Popen(
                 [binary_name, '-cfgdir', self._ba_root_path],
                 stdin=subprocess.PIPE,
                 cwd='dist')
+        except Exception as exc:
+            print(f'Error launching server subprocess: {exc}',
+                  file=sys.stderr,
+                  flush=True)
 
         # Do the thing.
         # No matter how this ends up, make sure the process is dead after.
+        if self._subprocess is not None:
             try:
                 self._run_subprocess_until_exit()
             finally: