cli/run: Require either --executable or --program

This prevents a crash when "bottles-cli run" is invoked without
an --executable or --program.

bottles/frontend/cli/cli.py

@@ -660,6 +660,10 @@ def run_program(self):
         bottle = mng.local_bottles[_bottle]
         programs = mng.get_programs(bottle)
 
+        if _executable is None and _program is None:
+            sys.stderr.write(f"Please specify either --executable or --program\n")
+            exit(1)
+
         if _program is not None:
             if _executable is not None:
                 sys.stderr.write(f"Cannot specify both --program and --executable\n")